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At least 19 recordsLinked to original sources
PubMed2002

Density-dependent birth rate, birth pulses and their population dynamic consequences.

In most models of population dynamics, increases in population due to birth are assumed to be time-independent, but many species reproduce only during a single period of the year. We propose a single-species model with stage structure for the dynamics in a wild animal population for which births occur in a single pulse once per time period. Using the discrete dynamical system determined by the stroboscopic map, we obtain an exact periodic solution of systems which are with Ricker functions or Beverton-Holt functions, and obtain the threshold conditions for their stability. Above this threshold, there is a characteristic sequence of bifurcations, leading to chaotic dynamics, which implies that the dynamical behaviors of the single species model with birth pulses are very complex, including small-amplitude annual oscillations, large-amplitude multi-annual cycles, and chaos. This suggests that birth pulse, in effect, provides a natural period or cyclicity that allows for a period-doubling route to chaos.

Animals↗
PubMed1979

Seasonal migration and seasonal variation in fecundability: effects on birth rates and birth intervals.

Seasonal patterns in conception rates have been documented in several recent studies. In this paper, a mathematical model of the reproductive process is developed through which the impact of such variation in conception rates is assessed. It is found that the effect on fertility can be quite substantial, but that the birth rate when seasonal variation is occurring is approximated well by the birth rate calculated when conception rates are constant at their mean. These results indicate that further documentation of seasonality in conception rates and exploration of the causes of these patterns and their change is an important area for demographic research.

Birth Rate↗
PubMed1999

Live-birth rates and multiple-birth risk using in vitro fertilization.

CONTEXT: To maximize birth rates, physicians who perform in vitro fertilization (IVF) often transfer multiple embryos, but this increases the multiple-birth risk. Live-birth and multiple-birth rates may vary by patient age and embryo quality. One marker for embryo quality is cryopreservation of extra embryos (if embryos are set aside for cryopreservation, higher quality embryos may have been available for transfer). OBJECTIVE: To examine associations between the number of embryos transferred during IVF and live-birth and multiple-birth rates stratified by maternal age and whether extra embryos were available (ie, extra embryos cryopreserved). DESIGN AND SETTING: Retrospective cohort of 300 US clinics reporting IVF transfer procedures to the Centers for Disease Control and Prevention in 1996. SUBJECTS: A total of 35554 IVF transfer procedures. MAIN OUTCOME MEASURES: Live-birth and multiple-birth rates (percentage of live births that were multiple). RESULTS: A total number of 9873 live births were reported (multiple births from 1 pregnancy were counted as 1 live birth). The number of embryos needed to achieve maximum live- birth rates varied by age and whether extra embryos were cryopreserved. Among women 20 to 29 years and 30 to 34 years of age, maximum live-birth rates (43 % and 36%, respectively) were achieved when 2 embryos were transferred and extra embryos were cryopreserved. Among women 35 years of age and older, live-birth rates were lower overall and regardless of whether embryos were cryopreserved, live-birth rates increased if more than 2 embryos were transferred. Multiple-birth rates varied by age and the number of embryos transferred, but not by whether embryos were cryopreserved. With 2 embryos transferred, multiple-birth rates were 22.7%, 19.7%, 11.6%, and 10.8% for women aged 20 to 29, 30 to 34, 35 to 39, and 40 to 44 years, respectively. Multiple-birth rates increased as high as 45.7% for women aged 20 to 29 years and 39.8% for women aged 30 to 34 years if 3 embryos were transferred. Among women aged 35 to 39 years, the multiple-birth rate was 29.4% if 3 embryos were transferred. Among women 40 to 44 years of age, the multiple-birth rate was less than 25% even if 5 embryos were transferred. CONCLUSIONS: Based on these data, the risk of multiple births from IVF varies by maternal age and number of embryos transferred. Embryo quality was not related to multiple birth risk but was associated with increased live-birth rates when fewer embryos were transferred.

Adult↗
PubMed2000

Selection of patients suitable for one-embryo transfer may reduce the rate of multiple births by half without impairment of overall birth rates.

The aim of the present study was to identify a subset of patients at high risk of multiple birth after IVF and hence suitable for one-embryo transfer, which undoubtedly would reduce the multiple birth rate. This retrospective study included 2107 IVF cycles in which two embryos were transferred. Factors with possible correlation to multiple birth were studied in a multivariate analysis. The factors included background data (female age, previous pregnancies and births, previous IVF cycles, indication for IVF) and IVF cycle characteristics. The following factors were independently predictive of multiple birth: female age expressed a negative correlation while number of good quality embryos transferred was positively correlated. A subset of patients was identified as being at high risk of multiple birth by including age, cycle number and presence of tubal infertility in a model derived from a logistic regression analysis. The rate of multiple births can be reduced from 26% to 13% of all births if one-embryo transfer is performed in selected cases. The total birth rate will decrease from 29% to 25% but may be completely restored by performing one additional one-embryo freeze transfer in high risk patients who do not achieve a term pregnancy.

Adult↗
PubMed2003

Live-birth rates and multiple-birth risk of assisted reproductive technology pregnancies conceived using thawed embryos, USA 1999-2000.

BACKGROUND: Increasing use of assisted reproductive technology treatments has been associated with the current rise in multiple births in the USA. Embryo cryopreservation and subsequent thawed embryo transfer may favourably impact the multiple-birth risk by relieving some pressure that patients and providers may feel to transfer several embryos in a single cycle. The study objective was to examine both live-birth rates and multiple-birth risk in thawed cycles. METHODS: The authors used a population-based sample of 21 555 assisted reproductive technology procedures performed in US clinics in 1999 and 2000 that used thawed embryos derived from the patient's oocytes. RESULTS: Both patient age and the number of embryos transferred were independent predictors of live birth. Even among women aged 20-29 years, the transfer of three embryos resulted in an increase in the live-birth rate compared with cycles in which one or two embryos were transferred. This increase in success was accompanied by an increased multiple-birth risk. In all age groups up to 40 years, the transfer of just two embryos resulted in a multiple-birth risk of 16-17%. The multiple-birth risk increased with the number of embryos transferred. CONCLUSIONS: Patient age and the number of embryos transferred significantly affect live-birth and multiple-birth rates among women who use thawed embryos.

Adult↗
PubMed1976

[Family formation by schizophrenic and manic-depressive patients according to an epidemiologic study (birth rate)].

The birth rate among patients with endogenous disorders is considered as an index of the degree of their social adaptation. It was established that in all forms of such disorders the level of birth rates and the rate of family formation is lower than that in the general population. However, in all endogenous disorders (except the malignant schizophrenia) there are some patients the adaptation of whom is not disturbed.

Adolescent↗
PubMed1987

Pregnancy and birth rates among sexually experienced US teenagers--1974, 1980, and 1983.

We examined pregnancy rates and birth rates among United States teenagers aged 15 to 19 years in 1974, 1980, and 1983. Pregnancy rate refers to live births plus induced abortions per 1000 women; birth rate refers to live births per 1000 women. We present these rates for all teens aged 15 to 19 years and for teens aged 15 to 19 years who were sexually experienced. Data sources included National Center for Health Statistics birth records, Centers for Disease Control abortion surveillance reports, and Bureau of the Census population estimates. Sexual experience estimates came from national surveys of adolescent sexual behavior. Between 1974 and 1980, the pregnancy rate among all teens increased; the pregnancy rate among sexually experienced teens declined. From 1980 to 1983, the pregnancy rate declined among all teens and among sexually experienced teens. Birth rates among US teenagers (all teens and sexually experienced teens) declined between 1974 and 1983. Whereas the decline in the birth rate from 1974 to 1980 was primarily due to increased use of abortion, the decline from 1980 to 1983 related to the decrease in teenage pregnancies.

Abortion, Induced↗
PubMed1974

[Birth rate].

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Birth Rate↗