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Steen Rasmussen

Publications and source records attributed to Steen Rasmussen.

14 recordsLinked to original sources
PubMed2006

Hyperdynamics for entropic systems: time-space compression and pair correlation function approximation.

We develop a generalized hyperdynamics method that is able to simulate slow dynamics in atomistic general (both energy- and entropy-dominated) systems. We show that a few functionals of the pair correlation function, involving two-body entropy, form a low-dimensional collective space, which is a good approximation that is able to distinguish stable and transitional conformations. A bias potential, which raises the energy in stable regions, is constructed on the fly. We examine the slow nucleation processes of a Lennard-Jones gas and show that our method can generate correct long-time dynamics without prior knowledge.

Journal Article↗
PubMed2006

Spontaneous preterm delivery in primiparous women at low risk in Denmark: population based study.

OBJECTIVES: To analyse trends in preterm delivery and the factors contributing to preterm delivery in Denmark. To construct a standard population at low risk (white European, 20-40 years of age, with a singleton spontaneous pregnancy) and describe the changes in this population so that time trends can be compared internationally. DESIGN: Population based study. PARTICIPANTS: 99.8% of all deliveries in Denmark, 1995-2004. MAIN OUTCOME MEASURES: Proportion of babies born at less than 37 weeks' completed gestation for each year in the overall population and in a standard population at low risk. RESULTS: Overall, the proportion of preterm deliveries increased by 22% from 1995 to 2004. During the same period, known risk factors for preterm delivery such as in vitro fertilisation, multiple pregnancies, and elective deliveries also increased, and logistic regression analyses showed that these factors were associated with an increased risk of preterm delivery. Spontaneous preterm deliveries in primiparous women at low risk rose 51% (from 3.8% to 5.7%) during this time compared with 20% (2.7% to 3.2%) in multiparous women at low risk. CONCLUSIONS: The overall proportion of preterm deliveries increased significantly from 1995 to 2004 and primiparity and multiple birth were the most important contributing factors. The rise in spontaneous preterm deliveries in the standard population of primiparous women at low risk was greater than in the total population.

Adult↗
PubMed2006

Experimentally tracing the key steps in the origin of life: The aromatic world.

Life is generally believed to emerge on Earth, to be at least functionally similar to life as we know it today, and to be much simpler than modern life. Although minimal life is notoriously difficult to define, a molecular system can be considered alive if it turns resources into building blocks, replicates, and evolves. Primitive life may have consisted of a compartmentalized genetic system coupled with an energy-harvesting mechanism. How prebiotic building blocks self-assemble and transform themselves into a minimal living system can be broken into two questions: (1) How can prebiotic building blocks form containers, metabolic networks, and informational polymers? (2) How can these three components cooperatively organize to form a protocell that satisfies the minimal requirements for a living system? The functional integration of these components is a difficult puzzle that requires cooperation among all the aspects of protocell assembly: starting material, reaction mechanisms, thermodynamics, and the integration of the inheritance, metabolism, and container functionalities. Protocells may have been self-assembled from components different from those used in modern biochemistry. We propose that assemblies based on aromatic hydrocarbons may have been the most abundant flexible and stable organic materials on the primitive Earth and discuss their possible integration into a minimal life form. In this paper we attempt to combine current knowledge of the composition of prebiotic organic material of extraterrestrial and terrestrial origin, and put these in the context of possible prebiotic scenarios. We also describe laboratory experiments that might help clarify the transition from nonliving to living matter using aromatic material. This paper presents an interdisciplinary approach to interface state of the art knowledge in astrochemistry, prebiotic chemistry, and artificial life research.

Hydrocarbons, Aromatic↗
PubMed2005

[Neurological late sequelae in twins born after in vitro fertilisation--secondary publication. A national cohort study].

Currently 4% of the Danish national birth cohort is born after in vitro fertilisation techniques and almost 40% of these children are twins. In this register-based national cohort study, twins from assisted conception (n = 3393) had a similar risk of neurological sequelae and cerebral palsy as their naturally conceived peers (n = 10.239) and assisted conceived singletons (n = 5130). Further children born after intracytoplasmic sperm injection (ICSI) had the same risk of neurological sequelae as children born after conventional IVF.

Adult↗
PubMed2004

Hospital care utilization of IVF/ICSI twins followed until 2-7 years of age: a controlled Danish national cohort study.

BACKGROUND: IVF/ICSI twins are likely to have a higher risk of prematurity associated with higher morbidity. The aim of this study was to assess the use of hospital care resources in IVF/ICSI twins on data retrieved until 2-7 years of child age. METHODS: National controlled cohort study on hospital admissions and surgical interventions in 3393 IVF twins, 10,239 spontaneously conceived twins and 5130 IVF singletons born between 1995 and 2000 in Denmark. Cross-linkage of data from the Danish IVF Registry and the National Patient Registry enabled us to identify children who were admitted to hospital or underwent an operation. RESULTS: The frequency of hospitalized children was 69.8, 69.6 and 49.8%, and of children who underwent a surgical intervention 10.6, 11.2 and 8.5% in IVF/ICSI twins, control twins and IVF/ICSI singletons respectively. Odds ratios (OR) (95% confidence intervals) of hospitalization in IVF/ICSI twins versus control twins and IVF/ICSI singletons were 1.04 (0.96, 1.14) and 2.44 (2.22, 2.63) and OR adjusted for year of birth, maternal age and parity were 1.00 (0.91, 1.11) and 2.38 (2.17, 2.63) respectively. Also for term birth infants, IVF/ICSI twins were more likely to be hospitalized than IVF/ICSI singletons: adjusted OR 1.37 (1.22, 1.51). Similar risk of a surgical procedure was observed in IVF/ICSI versus control twins. However, IVF/ICSI twins more often underwent a surgical intervention than IVF/ICSI singletons: adjusted OR 1.26 (1.08, 1.47). This risk disappeared when restricted to term infants: adjusted OR 1.00 (0.81, 1.22). Different sex IVF/ICSI and control twins had equal risk of admissions and surgical interventions, and ICSI children had the same risk as children born after conventional IVF. CONCLUSIONS: Though the use of hospital care resources was similar in IVF/ICSI and control twins, the over-use in IVF/ICSI twins versus IVF/ICSI singletons adds to the arguments for implementing elective single embryo transfer as our standard procedure.

Child↗
PubMed2004

Neurological sequelae in twins born after assisted conception: controlled national cohort study.

OBJECTIVE: To compare neurological sequelae in twins born after assisted conception with singletons after assisted conception and naturally conceived twins and to assess neurological sequelae in children conceived after in vitro fertilisation (IVF) compared with intracytoplasmic sperm injection (ICSI). DESIGN: Controlled, national register based, cohort study. PARTICIPANTS: Twins (n = 3393) and singletons (n = 5130) conceived by using assisted reproductive technologies and naturally conceived twins (n = 10 239) born in Denmark between 1995 and 2000. The children's age at time of follow up was 2-7 years. DATA SOURCES: Children were identified by cross linkage of the national medical birth registry and the national registry for in vitro fertilisation. Neurological and psychiatric diagnoses were retrieved from the national patients' registry and the Danish psychiatric central registry. MAIN OUTCOME MEASURES: Neurological sequelae, defined as cerebral palsy, mental retardation, severe mental developmental disturbances, and retarded psychomotor development. Further we made separate analyses on the specific cerebral palsy diagnosis. RESULTS: The crude prevalence rates per 1000 of neurological sequelae in twins and singletons after assisted conception and in naturally conceived twins were 8.8, 8.2, and 9.6, and of cerebral palsy 3.2, 2.5, and 4.0, respectively. In twins after assisted conception compared with control twins, the odds ratios of neurological sequelae and specifically of cerebral palsy, adjusted for child sex and year of birth, were 0.9 (95% confidence interval 0.6 to 1.4) and 0.8 (0.4 to 1.6), respectively. The corresponding odds ratios for twins after assisted conception compared with singletons after assisted conception were 1.1 (0.7 to 1.7) for neurological sequelae and 1.3 (0.6 to 2.9) for cerebral palsy. The odds ratio of neurological sequelae in children conceived by ICSI was 0.9 (0.5 to 1.7) nu children conceived by IVF. CONCLUSIONS: Twins from assisted conception have a similar risk of neurological sequelae as their naturally conceived peers and singletons from assisted conception. Children born after ICSI have the same risk of neurological sequelae as children born after IVF.

Cohort Studies↗
PubMed2004

Neonatal outcome in a Danish national cohort of 3438 IVF/ICSI and 10,362 non-IVF/ICSI twins born between 1995 and 2000.

BACKGROUND: In Denmark, one-third of twin pregnancies are the result of IVF/ICSI treatment. Limited data on neonatal outcome in IVF/ICSI twins are available in the literature. METHODS: A register study was conducted on neonatal morbidity and mortality in a complete national twin cohort including all 3438 (3393 live-born) IVF/ICSI and 10,362 (10,239 live-born) non-IVF/ICSI twins born between 1995 and 2000. Twins were identified in the National Medical Birth Registry and dichotomized into IVF/ICSI and non-IVF/ICSI by cross-reference with the Danish IVF Registry. Data on neonatal morbidity and mortality were retrieved from the Danish Patient Registry and the Danish Registry of Causes of Deaths. In order to exclude monozygotic twins, sub-analyses on unlike-sex twins were conducted. RESULTS: A birth weight discordance of >20% was observed in 20.6% of IVF/ICSI versus 15.7% of control twin pairs (P < 0.001). The risk of discordant birth weight >20% was OR 1.29 (95% CI 1.04-1.58) in unlike-sex IVF/ICSI twins versus control twins. The risk of delivery at <37 completed weeks and birth weight <2500 g was similar in the two cohorts; however, in unlike-sex IVF/ICSI versus control twins the risk of delivery at <37 weeks and birth weight <2500 g was OR 1.22 (95% CI 1.09-1.38) and OR 1.25 (1.11-1.40) respectively. After stratification for maternal age and parity, these risks disappeared. IVF/ICSI twins carried a higher risk of admittance to a neonatal intensive care unit (NICU) than control twins (OR 1.18, 95% CI 1.09-1.27), and this was even more pronounced in unlike-sex twins [OR 1.34 (95% CI 1.19-1.51)]. No differences were observed in malformation or mortality rates between the two cohorts. CONCLUSIONS: Despite higher birth weight discordance and more NICU admissions among IVF/ICSI twins, neonatal outcome in IVF/ICSI twins seems to be comparable with that of non-IVF/ICSI twins, when only dizygotic twins were considered in the comparisons.

Birth Weight↗
PubMed2003

The availability of perinatal health indicators in Europe.

This paper uses the results of the PERISTAT feasibility study to assess the extent to which the participating countries of Europe were able to provide data to construct the core and recommended indicators of perinatal health defined in the project. After describing the approaches used for data collection in participating countries, this paper describes the extent to which they were able to provide the data requested to construct the indicators. It documents data sources within each country and their characteristics. The paper then discusses influences on the agenda, particularly the extent to which data collection occurs as a by-product of other processes such as civil registration and the administration of health care and how these processes can both enable and impede data collection. It closes by suggesting how data collection in Europe can be improved in order to widen the scope of the agenda for compiling perinatal indicators.

Data Collection↗
PubMed2003

Collective intelligence of the artificial life community on its own successes, failures, and future.

We describe a novel Internet-based method for building consensus and clarifying conflicts in large stakeholder groups facing complex issues, and we use the method to survey and map the scientific and organizational perspectives of the artificial life community during the Seventh International Conference on Artificial Life (summer 2000). The issues addressed in this survey included artificial life's main successes, main failures, main open scientific questions, and main strategies for the future, as well as the benefits and pitfalls of creating a professional society for artificial life. By illuminating the artificial life community's collective perspective on these issues, this survey illustrates the value of such methods of harnessing the collective intelligence of large stakeholder groups.

Artificial Intelligence↗
PubMed2003

Bridging nonliving and living matter.

Assembling non-biological materials (geomaterials) into a proto-organism constitutes a bridge between nonliving and living matter. In this article we present a simple step-by-step route to assemble a proto-organism. Many pictures have been proposed to describe this transition within the origins-of-life and artificial life communities, and more recently alternative pictures have been emerging from advances in nanoscience and biotechnology. The proposed proto-organism lends itself to both traditions and defines a new picture based on a simple idea: Given a set of required functionalities, minimize the physicochemical structures that support these functionalities, and make sure that all structures self-assemble and mutually enhance each other's existence. The result is the first concrete, rational design of a simple physicochemical system that integrates the key functionalities in a thermodynamically favorable manner as a lipid aggregate integrates proto-genes and a proto-metabolism. Under external pumping of free energy, the metabolic processes produce the required building blocks, and only specific gene sequences enhance the metabolic kinetics sufficiently for the whole system to survive. We propose an experimental implementation of the proto-organism with a discussion of our experimental results, together with relevant results produced by other experimental groups, and we specify what is still missing experimentally. Identifying the missing steps is just as important as providing the road map for the transition. We derive the kinetic and thermodynamic conditions of each of the proto-organism subsystems together with relevant theoretical and computational results about these subsystems. We present and discuss detailed 3D simulations of the lipid aggregation processes. From the reaction kinetics we derive analytical aggregate size distributions, and derive key properties of the metabolic efficiency and stability. Thermodynamics and kinetics of the ligation directed self-replication of the proto-genes is discussed, and we summarize the full life cycle of the proto-organism by comparing size, replication time, and energy with the biomass efficiency of contemporary unicells. Finally, we also compare our proto-organism picture with existing origins-of-life and protocell pictures. By assembling one possible bridge between nonliving and living matter we hope to provide a piece in the ancient puzzle about who we are and where we come from.

Artificial Intelligence↗
PubMed2002

Urban growth simulation from "first principles".

General and mathematically transparent models of urban growth have so far suffered from a lack in microscopic realism. Physical models that have been used for this purpose, i.e., diffusion-limited aggregation, dielectric breakdown models, and correlated percolation all have microscopic dynamics for which analogies with urban growth appear stretched. Based on a Markov random field formulation we have developed a model that is capable of reproducing a variety of important characteristic urban morphologies and that has realistic microscopic dynamics. The results presented in this paper are particularly important in relation to "urban sprawl," an important aspect of which is aggressively spreading low-density land uses. This type of growth is increasingly causing environmental, social, and economical problems around the world. The microdynamics of our model, or its "first principles," can be mapped to human decisions and motivations and thus potentially also to policies and regulations. We measure statistical properties of macrostates generated by the urban growth mechanism that we propose, and we compare these to empirical measurements as well as to results from other models. To showcase the open-endedness of the model and to thereby relate our work to applied urban planning we have also included a simulated city consisting of a large number of land use classes in which also topographical data have been used.

Journal Article↗