PubMed · 7848192
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J S Robinson, R F Seamark, J A Owens. 1994. Placental function.. https://doi.org/10.1111/j.1479-828x.1994.tb01065.x
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OBJECTIVE: To develop a current national fetal growth curve that can be used as a common reference point by researchers to facilitate investigations of the predictors and consequences of small and large for gestational age delivery. METHODS: Single live births to United States resident mothers in 1991 (n = 3,134,879) were used for the development of this curve, which was compared with four previously published fetal growth curves. Techniques were developed to address cases with implausible birth weight-gestational age combinations and to smooth fetal growth curves across gestational age categories. RESULTS: In general, the previously published fetal growth curves underestimated the 1991 United States reference curve. This underestimation is most apparent during the latter weeks of gestation, approximately 33-38 weeks. CONCLUSION: Our findings indicate that the prevalence of fetal growth restriction (FGR) will vary markedly, depending on the fetal growth curve used. Furthermore, many previously published fetal growth curves no longer provide an up-to-date reference for describing the distribution of birth weight by gestational age and for determining FGR that is consistent with the most recent live birth data for the entire United States.
Many patients and obstetricians divide the events of human pregnancy into three intervals traditionally termed "trimesters." This system presumably arose from an equal division of the "9 months of pregnancy" into 3-month intervals. There are several problems with this system that follows pregnancy by months or trimesters. First, the average human pregnancy lasting 280 days or 40 weeks is not evenly divisible by three, leaving one to wonder how long each trimester is. Second, conversion from "weeks pregnant" to "months pregnant" is often an estimate that can foster misunderstanding between the patient and her obstetrician. Last, following pregnancy by the Gregorian calendar does not reflect embryonic or fetal developmental milestones. We propose a revision of this system to one in which natural embryonic and fetal developmental landmarks are used instead of trimesters to define the progressive stages of pregnancy. These landmarks occur approximately at 5-week intervals allowing a more simple division of pregnancy into four 10-week quartiles, each with two 5-week intervals. This article reviews many of these important landmarks within this framework. This system emphasizes a developmentally based way of understanding the events of pregnancy for both the patient and the obstetrician.