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PubMed1990

Freeze-thaw-induced changes of the zona pellucida explains decreased rates of fertilization in frozen-thawed mouse oocytes.

Frozen-thawed oocytes have a reduced rate of fertilization (48.8%) when compared with unfrozen controls (97%). In this study we have used zona-drilling to bypass the zona pellucida and investigate whether the decreased rate of fertilization is due to freezing-induced changes in the zona pellucida which prevent sperm penetration. After zona drilling the fertilization rate of frozen-thawed oocytes (87.8%) was the same as for zona-intact unfrozen controls (88%), indicating that freeze-thaw-induced changes at the level of the zona pellucida were responsible for the decreased rate of fertilization. To determine whether the changes were occurring during the manipulations before and after freezing or the complete freeze-thaw cycle, oocytes were exposed to the complete set of manipulations normally experienced during cryopreservation and appropriate control groups. A small but significant decrease in the rate of fertilization (82.8%) was apparent in oocytes exposed to the manipulations before and after freezing compared with controls (92.2%). The freeze-thaw-induced changes in the zona pellucida therefore occur primarily during the complete freeze-thaw cycle itself and not the manipulations before and after freezing and are responsible for the decreased rate of fertilization observed in frozen-thawed oocytes.

Animals↗

PubMed2003

Annual summary of vital statistics--2002.

The crude birth rate in 2002 was 13.9 births per 1000 population, the lowest ever reported for the United States. The number of births, the crude birth rate, and the fertility rate (64.8) all declined slightly (by 1% or less) from 2001 to 2002. Fertility rates were highest for Hispanic women (94.0), followed by black (65.4), Asian or Pacific Islander (63.9), Native American (58.0), and non-Hispanic white women (57.5). Fertility rates declined slightly for all race/ethnic groups from 2001 to 2002. The birth rate for teen mothers continued to fall, dropping 5% from 2001 to 2002 to 42.9 births per 1000 women aged 15 to 19 years, another record low. The teen birth rate has fallen 31% since 1991; declines were more rapid for younger teens aged 15 to 17 (40%) than for older teens aged 18 to 19 (23%). The proportion of all births to unmarried women remained approximately the same at one third. Smoking during pregnancy continued to decline; smoking rates were highest among teen mothers. In 2002, 26.1% of births were delivered by cesarean section, up 7% since 2001 and 26% since 1996. The primary cesarean rate has risen 23% since 1996, whereas the rate of vaginal birth after a previous cesarean delivery has fallen 55%. The use of timely prenatal care increased slightly to 83.8% in 2002. From 1990 to 2002, the use of timely prenatal care increased by 6% (to 88.7%) for non-Hispanic white women, by 24% (to 75.2%) for black women, and by 28% (to 76.8%) for Hispanic women, thus narrowing racial disparities. The percentage of preterm births rose to 12.0% in 2002, from 10.6% in 1990 and 9.4% in 1981. Increases were largest for non-Hispanic white women. The percentage of low birth weight (LBW) births also increased to 7.8% in 2002, up from 6.7% in 1984. Twin and triplet/+ birth rates both increased by 3% from 2000 to 2001. Multiple births accounted for 3.2% of all births in 2001. The infant mortality rate (IMR) was 6.9 per 1000 live births (provisional data) in 2002 compared with 6.8 in 2001 (final data). The ratio of the IMR among black infants to that for white infants was 2.5 in 2001, the same as in 2000. Racial differences in infant mortality remain a major public health concern. The role of LBW in infant mortality remains a major issue. New Hampshire, Utah, and Massachusetts had the lowest IMRs. State-by-state differences in IMR reflect racial composition, the percentage of LBW, and birth weight-specific neonatal mortality rates for each state. The United States continues to rank poorly in international comparisons of infant mortality. Expectation of life at birth reached a record high of 77.2 years for all sex and race groups combined in 2001. Death rates in the United States continue to decline. Between 2000 and 2001, death rates declined for the 3 leading causes of death: diseases of the heart, malignant neoplasms, and cerebrovascular diseases. Death rates for children ages 1 to 19 years decreased for unintentional injuries by 3.3% in 2001; the death rate for chronic lower respiratory diseases decreased by 25% in 2001. Cancer and suicide levels did not change for children ages 1 to 19. A large proportion of childhood deaths continue to occur as a result of preventable injuries.

Adolescent↗

PubMed1995

High levels of sperm-associated antibodies impair human spermoolemma interaction after subzonal insemination.

OBJECTIVE: To investigate the impact of sperm-associated antibodies in the fertilization process beyond the zona pellucida in human oocytes. DESIGN: Subzonal insemination (SUZI) was performed with antibody-coated spermatozoa from patients with autoimmune infertility. Sperm parameters and antibody binding were analyzed after Percoll selection and were compared with SUZI outcome. PATIENTS: Patients (n = 29) with an immunobead test higher than 60% for immunoglobulin (Ig)G, IgA, or both classes were included in the study (38 attempts). Twenty-six patients had previous IVF failures, and the semen characteristics for three were unsuitable for IVF. RESULTS: Diploid fertilization occurred in 52.6% of cycles and 16.0% of oocytes (n = 263). The fertilization rate was inversely correlated to the proportion of spermatozoa coated with IgG in the Percoll-selected sperm suspension. Three of the 16 attempts in which > 90% of the spermatozoa were coated with IgG resulted in a low diploid rate of 4.1%. Fertilization was obtained in 18 of the 22 attempts in which < 90% of spermatozoa were IgG coated, with a diploid fertilization rate of 22.8%. Association of both classes of antibodies (IgG and IgA) further impaired the fertilization rate. Twenty ETs gave four pregnancies: one biochemical, one ongoing, and two giving birth to healty babies (1 singleton and 2 twins). CONCLUSIONS: Subzonal insemination is a potential solution for achieving fertilization in cases of IVF failure because of sperm autoimmunity. However, IgG inhibits in a dose-dependent manner the fusion of gametes' membranes. Though the level of IgA antibodies does not appear as critical as that of IgG antibodies, association of both classes of Igs impairs the probability of fertilization.

Adult↗

PubMed1999

Births: final data for 1997.

OBJECTIVES: This report presents 1997 data on U.S. births according to a wide variety of characteristics. Data are presented for maternal demographic characteristics including age, live-birth order, race, Hispanic origin, marital status, and educational attainment; maternal lifestyle and health characteristics (medical risk factors, weight gain, and tobacco and alcohol use); medical care utilization by pregnant women (prenatal care, obstetric procedures, complications of labor and/or delivery, attendant at birth, and method of delivery); and infant health characteristics (period of gestation, birthweight, Apgar score, abnormal conditions, congenital anomalies, and multiple births). Also presented are birth and fertility rates by age, live-birth order, race, Hispanic origin, and marital status. Selected data by mother's State of residence are shown including teenage birth rates and total fertility rates, as well as data on month and day of birth, sex ratio, and age of father. Trends in fertility patterns and maternal and infant characteristics are described and interpreted. METHODS: Descriptive tabulations of data reported on the birth certificates of the 3.9 million births that occurred in 1997 are presented. RESULTS: Birth and fertility rates declined very slightly in 1997. Birth rates for teenagers fell 3 to 5 percent. Rates for women in their twenties changed very little, whereas rates for women in their thirties rose 2 percent. The number of births and the birth rate for unmarried women each declined slightly in 1997 while the percent of births that were to unmarried women was unchanged. Smoking by pregnant women overall dropped again in 1997, but continued to increase among teenagers. Improvements in prenatal care utilization continued. The cesarean delivery rate increased slightly after declining for 7 consecutive years. The proportion of multiple birth continued to rise; higher order multiple births (e.g., triplets, quadruplets) rose by 14 percent in 1997, following a 20 percent rise from 1995 to 1996. Key measures of birth outcome--the percents of low birthweight and preterm births--increased, with particularly large increases in the preterm rate. These changes are in large part the result of increases in multiple births.

Adolescent↗

PubMed2000

Births: final data for 1998.

OBJECTIVES: This report presents 1998 data on U.S. births according to a wide variety of characteristics. Data are presented for maternal demographic characteristics including age, live-birth order, race, Hispanic origin, marital status, and educational attainment; maternal lifestyle and health characteristics (medical risk factors, weight gain, and tobacco and alcohol use); medical care utilization by pregnant women (prenatal care, obstetric procedures, complications of labor and/or delivery, attendant at birth, and method of delivery); and infant health characteristics (period of gestation, birthweight, Apgar score, abnormal conditions, congenital anomalies, and multiple births). Also presented are birth and fertility rates by age, live-birth order, race, Hispanic origin, and marital status. Selected data by mother's State of residence are shown including teenage birth rates and total fertility rates, as well as data on month and day of birth, sex ratio, and age of father. Trends in fertility patterns and maternal and infant characteristics are described and interpreted. METHODS: Descriptive tabulations of data reported on the birth certificates of the 3.94 million births that occurred in 1998 are presented. RESULTS: Birth and fertility rates increased in 1998 by about 1 percent, the first increase since 1990. Birth rates for teenagers fell 2-5 percent. Rates for women in their twenties increased 1-2 percent each, whereas rates for women in their thirties rose 2-4 percent. All measures of childbearing by unmarried women increased in 1998; the number of births rose 3 percent, the birth rate increased about 1 percent while the percent of births that were to unmarried women rose to 32.8 percent. Smoking by pregnant women overall dropped again in 1998, but continued to increase among teenagers. Improvements in prenatal care utilization continued. The cesarean delivery rate increased for the second year after declining for 7 consecutive years. The proportion of multiple births continued to rise; higher order multiple births (e.g., triplets, quadruplets) rose by 13 percent in 1998, following a 14 percent rise from 1996 to 1997. Key measures of birth outcome--the percents of low birthweight and preterm births--increased. These changes are in large part the result of increases in multiple births.

Adolescent↗

PubMed2003

Correlation between human follicular diameter and oocyte outcomes in an ICSI program.

PURPOSE: To determine the correlation between the follicular sizes and oocyte recovery, metaphase II oocyte recovery, fertilization rate and good embryo quality from mature and immature oocytes in an intracytoplasmic sperm injection (ICSI) program. METHODS: 991 follicles obtained from 72 ICSI cycles were classified into three groups according to their diameters as measured by transvaginal ultrasound including group A (< 10 mm), group B (10-14 mm), and group C (> 14 mm). All obtained oocytes were classified according to their nuclear maturation: germinal vesicle (GV), metaphase I (MI) and metaphase II (MII). Mature oocytes underwent ICSI while immature oocytes were further cultured until maturity before ICSI was performed. The rates of fertilization and good quality embryos at day 3 were evaluated. RESULTS: A progressive and significant increase in the rates of oocyte recovery and MII oocyte recovery were observed from group A follicles compared to the other groups (p < 0.001). The fertilization rate of mature and in vitro matured oocytes, as well as the rate of good quality embryos showed a tendency to increase from group A to group C follicles, but not significantly. The corresponding fertilization rates were 78 and 55.3% (p < 0.001) for mature and in vitro matured oocytes, respectively. CONCLUSION: Collection of oocytes from small follicles, especially with a mean diameter less than 10 mm, and in vitro maturation of immature oocytes before fertilization may allow the total number of good quality and transferable embryos to be increased.

Adult↗

PubMed1993

Effects of bull and heparin and sperm concentrations on in vitro fertilization of buffalo (Bubalus bubalis ) oocytes matured in vitro.

A study was undertaken to assess the ability of spermatozoa from 6 buffalo bulls, at different levels of heparin and sperm concentrations, to achieve an acceptable level of fertilization in vitro. Frozen-thawed spermatozoa, 3 dosages of heparin (0, 10 and 100 ug/ml) in the presence and absence of penicillamine, hypotaurine and epinephrine (PHE), and 4 sperm concentrations (1 x 10(6), 2 x 10(6), 3 x 10(6) and 4 x 10(6) /ml) were studied using 3202 buffalo oocytes. The mean proportions of fertilized oocytes in the group treated with 10 ug/ml of heparin were significantly higher (P<0.05) with the semen of Bulls A, B and C (44.7 to 64.3%) than in medium devoid of heparin. An increase in the dosage of heparin from 10 ug/ml to 100 ug/ml reduced the overall fertilization rate. However, optimal fertilization (30.9%) at 100 ug/ml heparin was observed for semen from Bull D. Bulls E and F yielded the lowest fertilization rate (9.6 and 14.2%, respectively) at the above mentioned heparin dosage. Analysis of sperm density revealed that a concentration of 2 x 10(6) spermatozoa yielded optimal fertilization rates in vitro. Higher sperm concentrations (3 x 10(6) or 4 x 10(6)) resulted in higher oocyte penetration rates but gave rise to polyspermy.

Journal Article↗

PubMed1987

The effect of nitrous oxide on in vitro fertilization success rate.

The authors studied the effect of nitrous oxide on success rates for in vitro fertilization and pregnancy in women undergoing laparoscopy for oocyte retrieval. Ninety-eight patients in an in vitro fertilization program were randomly assigned to an anesthetic regimen including either 0.7% (end-tidal) isoflurane with 60% nitrous oxide in oxygen, or 1.4% (end-tidal) isoflurane in oxygen. Success rates for fertilization and pregnancy in 44 additional patients who declined randomization were also studied. Among the 51 randomized patients who did not receive nitrous oxide, 192 oocytes were obtained and 122 fertilized (63.5%), resulting in eight pregnancies (16.3%). From the 47 randomized patients given nitrous oxide, 168 oocytes were retrieved and 114 fertilized (67.9%), resulting in nine pregnancies (19.1%). No significant differences between rates of fertilization or pregnancy emerged between groups. Such differences would have been found with an 80% probability had nitrous oxide had a 20% effect on oocyte fertilization.

Female↗

PubMed1989

Predictive value of sperm morphology and movement characteristics in the outcome of in vitro fertilization of human oocytes.

One hundred fourteen semen samples from Chinese males were analyzed for routine semen parameters including the semen volume, sperm count, percentage motility, and percentage normal morphology. Of these 114 samples, 54 also had movement characteristics of seminal and swim-up sperm evaluated by the computer image analyzer system (Cellsoft; Cryo Resources Co., New York). All semen samples were subjected to the swim-up procedure to harvest the motile sperm before inseminations of human oocytes. Fertilization was considered to have occurred when at least one oocyte was observed with two or more pronuclei. Semen samples were classified as infertile (0% fertilization rate; N = 32) or fertile (greater than 0% fertilization rate; N = 82) before statistical analyses. There was a significant difference (P less than 0.005) in percentage normal morphology of seminal sperm between the fertile (mean +/- SE; 67.3 +/- 1.2%) and the infertile (59.3 +/- 2.2%) samples. The percentage normal morphology of seminal sperm correlated (r = 0.3049; P less than 0.002) with the fertilization rate and this parameter was selected by the multivariate stepwise discriminant analysis as the discriminator capable of predicting the fertilization rate with 57.9% accuracy. Statistical analyses of samples where sperm movement was also evaluated demonstrated that there was significant differences (P less than 0.01) between the fertile (N = 38) and the infertile (N = 16) samples in percentage normal morphology of seminal sperm (67.8 +/- 1.8% vs 56.2 +/- 2.6%) and curvilinear velocity of swim-up sperm (89.2 +/- 3.5 vs 68.2 +/- 7.2 microns/sec).(ABSTRACT TRUNCATED AT 250 WORDS)

Female↗

PubMedDate not supplied

Encouraging rates of fertility after ectopic pregnancy.

A follow-up study (4 to 8 years) on fertility of 110 patients seeking pregnancy after ectopic pregnancy (EP) showed normal delivery in 65%, recurrent EP in 20%, and infertility in 15%. Delivery rates were better in patients who were under age 30 years, used an IUD at the time of operation for EP, and underwent conservative operative treatment for acute EP. The subsequent intrauterine pregnancy rates of EP patients who had used an IUD at the time of ectopic nidation were the same (92%) as after removal of IUD in the healthy population. It appears that IUD plays a predisposing role in EP only when in use. The study also shows that modern diagnostic and therapeutic procedures are beneficial for improving subsequent fertility rates after EP. These fertility rates are highly encouraging with respect to the massive increase of EP in the Finnish population.

Adult↗

PubMed2002

In vitro fertilization of in vitro-matured equine oocytes: effect of maturation medium, duration of maturation, and sperm calcium ionophore treatment, and comparison with rates of fertilization in vivo after oviductal transfer.

Three experiments were conducted to evaluate the effect of oocyte and sperm treatments on rates of in vitro fertilization (IVF) in the horse and to determine the capacity of in vitro-matured horse oocytes to be fertilized in vivo. There was no effect of duration of oocyte maturation (24 vs. 42 h) or calcium ionophore concentration during sperm capacitation (3 microM vs. 7.14 microM) on in vitro fertilization rates. Oocytes matured in 100% follicular fluid had significantly higher fertilization (13% to 24%) than did oocytes matured in maturation medium or in 20% follicular fluid (0% to 12%; P < 0.05). There was no significant difference in fertilization rate among 3 sperm treatments utilizing 7.14 microM calcium ionophore (12% to 21%). Of in vitro-matured oocytes recovered 40-44 h after transfer to the oviducts of inseminated mares, 77% showed normal fertilization (2 pronuclei to normal cleavage). Cleavage to 2 or more cells was seen in 22% of oocytes matured in follicular fluid and 63% of oocytes matured in maturation medium; this difference was significant (P < 0.05). We conclude that in vitro-matured horse oocytes are capable of being fertilized at high rates in the appropriate environment and that in vitro maturation of oocytes in follicular fluid increases fertilization rate in vitro but reduces embryo development after fertilization in vivo. Further work is needed to determine the optimum environment for sperm capacitation and IVF in the horse.

Animals↗

PubMed1996

Successful intracytoplasmic sperm injection without performing cytoplasmic aspiration.

OBJECTIVE: To compare the effect of cytoplasmic aspiration versus no aspiration before intracytoplasmic sperm injection (ICSI) on the rate of oocyte damage, fertilization rate, and embryo quality. DESIGN: A randomized prospective study on sibling oocytes. SETTING: The Egyptian IVF-ET Center, Cairo, Egypt. PARTICIPANTS: Fifty-eight patients who were infertile due to male factor who underwent 60 ICSI cycles. INTERVENTION: Intracytoplasmic sperm injection was performed on randomly allocated metaphase II oocytes with cytoplasmic aspiration in group I and without cytoplasmic aspiration in group II before sperm injection. MAIN OUTCOME MEASURE: Fertilization rate, oocyte damage rate, and embryo quality. RESULTS: Normal fertilization rate per injected oocyte was 61.4% in group I compared with 62.5% in group II. The damage rate per injected oocyte was 16.8% in group I compared with 4.6% in group II. Grade I embryos were 24.5% in group I compared with 48.5% in group II. CONCLUSION: Cytoplasmic aspiration before sperm injection in ICSI is not essential for oocyte activation. It did not improve the rate of normal fertilization. On the other hand, it increased the damaged oocyte rate and the rate of cytoplasmic fragments.

Adult↗

PubMedDate not supplied

Evaluation of the percentage of sperm motility at 24 h and sperm survival ratio for prediction of in vitro fertilization.

The present study was carried out to investigate the predictive value of the percentage of sperm motility after 24-h incubation and sperm survival ratio from semen and inseminated sperm suspension using grading of motility by WHO criteria with respect to the fertilization of oocytes in vitro. A total of 789 oocytes from 85 cases were inseminated and the mean fertilization rate obtained was 72.5%. There was no significant correlation between all of the sperm motility results with fertilization rate in vitro. All sperm motility results were not significantly different between the non-fertilizing group and the fertilizing group and also between the group of fertilization rate < or = 25th percentile (fertilization rate < or = 62.5%) and the group of fertilization rate > 62.5%. However, the initial percentage of rapid progressive sperm motility and progressive motility in semen and inseminated sperm suspension at 24 h gave significant differences between the group of fertilization < or = 50th percentile (fertilization rate < or = 80%) and the group of fertilization rate > 80%. Overall accuracy using these parameters for prediction of fertilization rate > 80% was only about 60%. In conclusion, the percentage of sperm motility at 24 h and sperm survival ratio in both semen and inseminated sperm suspension have no practical value in predicting fertilization rate in vitro. Moreover, detailed motility grading cannot improve the predictivity of these sperm motility parameters.

Cell Survival↗

PubMed1989

Relationship of bull fertility with daughter fertility and production traits in Holstein dairy cattle.

The phenotypic and genetic correlations between fertility ratings of AI bulls for conception rate and their estimated breeding values for daughters' fertility and production traits were calculated. Genetic correlations between fertility ratings of bulls for conception and heifer fertility traits (age at first breeding, age at last breeding, and number of insemination per conception) were negative and ranged from -.04 to -.23, indicating daughters of bulls with high fertility ratings were younger at first breeding and required fewer services to conceive. In general, genetic correlations between fertility ratings of bulls for conception rate and cow fertility traits (days from calving to first breeding, days open, and number of inseminations per conception) and production traits (breed class average milk and fat and fat percentage) in the first two lactations were also moderate to high and in the favorable direction. Although heritability of both male and female fertility is low, these data indicate that heavy use of sires with high fertility ratings could have a mild positive effect on both male and female fertility. Evidence is also found to indicate that in this breed, selection for increased milk yield should not impair genetic ability of cows to reproduce.

Animals↗

PubMed2006

Vitrification of mouse oocyte using open pulled straws compared with needles.

OBJECTIVE: To compare the survival rate of mouse oocytes and fertilization rate between using open pulled straws (OPS) and needles for vitrification. MATERIAL AND METHOD: Meiosis II oocytes from female C57B/6J mice aged 7-8 weeks were collected and allocated to two groups for vitrification by using OPS or needles. Vitrified oocytes were thawed, morphological survival and fertilization rate were examined. RESULTS: There was no obvious difference between the morphological survival rates of vitrified mouse oocytes using OPS and needles (66.7% vs 64.8%). Proportions Difference 1.9% (95% CI -7.1, 10.7). The vitrified oocytes from the needle had significantly higher percentages of fertilization rate than OPS (76.8% vs 62.5%). Proportions Difference -14.3% (95% CI -24.5, -3.6). CONCLUSION: Vitrification method of mouse oocytes using needles when compared to OPS provides a similar morphological survival rate and higher fertilization rate.

Animals↗

PubMed1996

Polyploidy and failed fertilization in in-vitro fertilization are related to patient's age and gamete quality.

A review of 392 cycles of in-vitro fertilization (IVF) was carried out in order to identify whether any factors such as sperm concentration at insemination, sperm motility or morphology, oocyte grading, number of oocytes retrieved, patient age, follicular stimulation protocols or duration of follicular growth, could be associated with the incidence of polyploidy or completely failed fertilization. The majority of polypronuclear fertilizations occurred in mature oocytes and in patients < 37 years of age and the incidence of polyploidy was strongly associated with fertilization and pregnancy rates. Fertilization rates were highest with mature oocytes. A significant linear trend was observed with failed fertilization and sperm concentration, morphology and motility. High rates of failed fertilization were found in patients > 37 years of age and with one to five oocytes retrieved. No significant difference was seen in stimulation protocols and oocyte grading between oocytes that fertilized and those that did not. Cycles with good sperm morphology and motility, the presence of mature oocytes, the retrieval of a large number of oocytes and younger maternal age seem to provide the best chance for IVF success.

Adult↗

PubMed1993

Pregnancy after subzonal insemination with spermatozoa lacking outer dynein arms.

The absence of outer dynein arms in the sperm flagellum induces an abnormal movement pattern associated with male infertility. These spermatozoa can decondense in zona-free hamster oocytes but result in a very low fertilization rate in in vitro fertilization. We hypothesized that subzonal insemination could help achieve fertilization and pregnancy. A randomized prospective trial (five couples, five cycles) comparing subzonal insemination (n = 31 oocytes) and routine IVF (n = 23 oocytes) was carried out. Oocytes were microinjected with 8.5 +/- 3.6 spermatozoa. In a second series (nine cycles), all the oocytes were microinjected with 10.5 +/- 4.3 spermatozoa. In the randomized series, the fertilization rate was 16.1% without polyploidy, whereas no fertilization was obtained after control IVF insemination. In the second series involving nine couples, six of whom were included in the first series, the fertilization rate increased to 57.8% with a 27.8% polyspermic rate. Eighty-eight per cent of the zygotes cleaved normally (29 out of 33). A total of 11 embryo transfers resulted in three pregnancies, one of which terminated one month later, a second being ongoing and the third delivering a healthy girl. A 21.4% pregnancy rate per cycle, with a 37.5% pregnancy rate per couple, justifies the use of subzonal insemination to treat this particular flagellar dyskinesia.

Adult↗

PubMed2003

Temporal differences in the regional twinning rates in Sweden after 1750.

When twinning rates are studied, maternal age and parity should be considered. Data on parity are seldom available. We studied information about the mean parity, using the gross reproduction rate, the total fertility rate and the crude birth rate. These are strongly correlated with the mean parity. The crude birth rate is more readily available than the gross reproduction rate or the total fertility rate. Earlier studies have shown that it is difficult to model variations in the twinning rate with data for the macrolevel. In this study these findings are explained by theoretical analyses and illustrated by empirical data. Sweden, having the oldest continuous population statistics and high twinning rates, offers excellent possibilities for analyses of the twinning rate. We considered data for the counties of Gotland, Alvsborg and Stockholm and the city of Stockholm from 1749 to 1960 and for Sweden until 1996. For Alvsborg, the twinning rate was low for the whole period, showing no statistically significant decreasing trend. It is mainly about 11-14 per thousand, which is only 50-60% of the twinning rate in Gotland in the 18th century. In Gotland, in the county of Stockholm, in the city of Stockholm and in Sweden as a whole, the decreasing trends in the twinning rate were statistically significant. The decreasing twinning rates converge towards the low twinning rate of Alvsborg. After standardization of the twinning rate, the differences remained and the low rate in Alvsborg could not be explained by maternal age.

Birth Rate↗