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Why is the rate of teen childbearing is so unusually high in the United States as a whole, and in some U.S. states in particular? U.S. teens are two and a half times as likely to give birth as compared to teens in Canada, around four times as likely as teens in Germany or Norway, and almost ten times as likely as teens in Switzerland. A teenage girl in Mississippi is four times more likely to give birth than a teenage girl in New Hampshire—and 15 times more likely to give birth as a teen compared to a teenage girl in Switzerland. We examine teen birth rates alongside pregnancy, abortion, and “shotgun” marriage rates as well as the antecedent behaviors of sexual activity and contraceptive use. We demonstrate that variation in income inequality across U.S. states and developed countries can explain a sizable share of the geographic variation in teen childbearing. Our reading of the totality of evidence leads us to conclude that being on a low economic trajectory in life leads many teenage girls to have children while they are young and unmarried. Teen childbearing is explained by the low economic trajectory but is not an additional cause of later difficulties in life. Surprisingly, teen birth itself does not appear to have much direct economic consequence. Our view is that teen childbearing is so high in the United States because of underlying social and economic problems. It reflects a decision among a set of girls to “drop-out” of the economic mainstream; they choose nonmarital motherhood at a young age instead of investing in their own economic progress because they feel they have little chance of advancement.

We estimate the effects of temperature shocks on birth rates in the United States between 1931 and 2010. We find that days with a mean temperature above 80°F cause a large decline in birth rates 8 to 10 months later. Unlike prior studies, we demonstrate that the initial decline is followed by a partial rebound in births over the next few months, implying that populations mitigate some of the fertility cost by shifting conception month. This shift helps explain the observed peak in late-summer births in the United States. We also present new evidence that hot weather most likely harms fertility via reproductive health as opposed to sexual activity. Historical evidence suggests that air conditioning could be used to substantially offset the fertility costs of high temperatures.

Women in the United States are much more likely to become mothers as teens than those in other rich countries. Teen births are particularly likely to be reported as unintended, leading to debate over whether better information on sex and contraception might lead to reductions in teen births. We contribute to this debate by providing causal evidence at the population level. Our causal identification strategy exploits county-level variation in the timing and receipt of federal funding for more comprehensive sex education and data on age-specific teen birth rates at the county level constructed from birth certificate natality data covering all births in the United States. Our results show that federal funding for more comprehensive sex education reduced county-level teen birth rates by more than 3%. Our findings thus complement the mixed evidence to date from randomized control trials on teen pregnancies and births by providing population-level causal evidence that federal funding for more comprehensive sex education led to reductions in teen births.

Background The landscape of assisted reproductive technology (ART) has seen a significant shift towards frozen-thawed embryo transfers (FET) over fresh transfers, driven by technological advancements and clinical considerations. This study aimed to compare live birth outcomes between primary FET and fresh transfers, focusing on cycles without preimplantation genetic testing (PGT), using United States national data from the SART CORS database spanning from 2014 to 2020. Methods We performed a retrospective cohort study of autologous first ART cycles without PGT comparing primary embryo transfer (frozen thaw vs. fresh) success rates from the 2014–2020 SARTCORS database. Live-birth rates (LBR) and cumulative live-birth rates (CLBR) were compared between first FET versus first fresh embryo transfer from an index retrieval. Multivariate logistic regression (MLR) determined association between live birth outcomes and method of transfer. In a subsequent sub-analysis, we compared these two embryo transfer methods among patients with either diminished ovarian reserve (DOR) or male factor infertility. Results 228,171 first ART cycles resulted in primary embryo transfer. 62,100 initial FETs and 166,071 fresh transfers were compared. Initial FETs demonstrated higher LBR and CLBR compared to fresh transfers (LBR 48.3% vs. 39.8%, p  < 0.001; CLBR 74.0% vs. 60.0%, p  < 0.0001). MLR indicated greater chances of live birth with FET across all age groups, with adjusted odds ratio (aOR) of live-birth incrementally increasing with advancing age groups. For DOR cycles, LBR and CLBR were significantly higher for FET compared to fresh (33.9% vs. 26.0%, p  < 0.001, 44.5% vs. 37.6%, p  < 0.0001), respectively. MF cycles also demonstrated higher LBR and CLBR with FET (52.3% vs. 44.2%, p  < 0.001, 81.2% vs. 68.9%, p  < 0.0001), respectively. MLR demonstrated that in DOR cycles, initial FET was associated with greater chance of live birth in age groups ≥ 35yo ( p  < 0.01), with aOR of live birth increasingly considerably for those > 42yo (aOR 2.63, p  < 0.0001). Conclusions Overall LBR and CLBR were greater for first FET than fresh transfers with incremental increases in odds of live birth with advancing age, suggesting the presence of a more favorable age-related change in endometrial receptivity present in frozen-thawed cycles. For both DOR and MF cycles, LBR and CLBR after primary transfer were greater for first FET than fresh. However, this was particularly evident in older ages for DOR cycles. This suggests that supraphysiologic stimulation in older DOR cycles may be detrimental to endometrial receptivity, which is in part corrected for in FET cycles.

The continuous declining rate of birth may cause severe worldwide problems, including population ageing and demographic imbalance. This paper aims to identify the geographic disparities and potential influential factors of the extreme downward excess of birth rates in the U.S. at the county level from 2003 to 2020. The innovative Bayesian generalized Pareto (GP) quantile regression model is employed for analysing the spatiotemporal pattern of stressful low birth rate and its association with socioeconomic, demographic, and meteorological factors. The optimal stochastic partial differential equation and autoregressive model of order one (SPDE-AR(1)) spatiotemporal structured model is selected based on common Bayesian criteria and scaled threshold weighted continuous ranked probability score, indicating apparent spatial unbalance and deteriorating tendency of falling birth rate. The findings show that elderly, highly educated population structure, city-developed situation, average birth weight, and average age of mother have a significant impact on the stressful downward excess of birth rate, particularly in examples of Charlotte, Sarasota, Collier counties in Florida, and Marin, Santa, Cruuz, San Mateo in California. Conversely, the increasing female proportion and GDP per capita can alleviate the declining birth rate pressure. Our methodology is productive for analysing extreme population stress including but not limited to birth weight and cognitive impairment.

The study examined the adult sex ratio and fertility rate at Birhan Health and Demographic Surveillance System (HDSS) in the Amhara region of Ethiopia. Globally, the sex ratio at birth and total population remains stable, with approximately 105.6 boys born for every 100 girls and 101 males born for every 100 females, respectively. Ethiopia’s average population sex ratio is 101 males to 100 females. Fertility rates have declined globally since 1950, including in sub-Saharan Africa. Ethiopia’s fertility rate decreased from 6.4 in 1990 to 4.6 in 2016. The HDSS monitors health and demographic conditions in both rural and urban areas, providing an updated sampling frame for nested studies. We used both the HDSS and the open cohort data to calculate key indicators, including Crude Birth Rate (CBR), General Fertility Rate (GFR), Total Fertility Rate (TFR), and Adult Sex Ratio (ASR). The study’s strength lies in its comprehensive approach to pregnancy screening and birth outcome registration, yielding valuable data beyond traditional surveys. The mid-year population was 72,776, with a higher number of males (38,454) than females (34,322). Among individuals aged 15–24, women comprised 37.4% of the group, resulting in a sex ratio of 167.4 males per 100 females. For the broader reproductive age range (15–50 years), the sex ratio was 125 males per 100 females. In 2022, the TFR was 3.44 children per woman, reflecting a 25.22% decline compared to Ethiopia’s national TFR of 4.6 in 2016. Other key fertility indicators also demonstrated notable reductions: the CBR was 19.98 per 1000 population, and the GFR was 104.78 per 1000 women of reproductive age, marking decreases of 37.24% and 32.83%, respectively, from the 2016 national averages. The findings indicate a youthful population with a higher male-to-female ratio, particularly among younger age groups. Fertility rates are notably lower compared to national figures. The decline may be attributed to gendered migration patterns and reduced conception risk among migrants, influenced by improved living conditions in urban areas and temporary separation from partners.

A study investigating the impact of COVID-19 on birth rates across women's age groups has not been previously conducted in Japan. Therefore, we examined this issue using national birth data. The analysis utilized monthly live birth data from the Vital Statistics from 2015 to 2023, which were accessed on July 27, 2025. We estimated the expected number of births in the post-pandemic period through quasi-Poisson regression analysis and calculated the ratios of the sum of actual births to that of predicted births for each women's age group. In addition, a segmented regression analysis was performed to assess the effect of the pandemic on birth rates, focusing on changes in level and trend across age groups. The ratios of the sum of actual births to that of predicted births in the post-pandemic period was close to one overall, but actual numbers were significantly lower than predicted numbers in women aged 15-19, 20-24, 25-29, and 35-39 years. Segmented regression analysis revealed significant declines in birth rates among women aged 15-19, 20-24, and 45-49 years due to the pandemic, with the rate ratios (RRs) being 0.880 (95% confidence interval (CI): 0.828, 0.935), 0.936 (95%CI: 0.907, 0.967), and 0.890 (95%CI: 0.824, 0.961), respectively, indicating that an immediate effect of the pandemic was evident in those age groups. In addition, significant trend changes were observed among women aged 20-24, 25-29, 35-39, and 45-49 years, with the RRs being 0.995 (95%CI: 0.993, 0.996), 0.998 (95%CI: 0.996, 0.999), 0.998 (95%CI: 0.997, 1.000), and 1.004 (95%CI: 1.001, 1.008), respectively, indicating an acceleration of the declining trend in some age groups. These findings suggest that the pandemic's effect on birth rates varies by women's age, with a more pronounced decrease among younger women.

Background Assisted Reproductive Technology (ART) has undergone considerable changes over the last decade, with consequences on ART outcomes in different regions of the world being unknown. Methods We conducted a systematic review of published national and regional ART registry data to assess how changes in clinical practice between 2004 and 2013 have impacted outcomes in Australia and New Zealand, Canada, Continental Europe, the United Kingdom (U.K.), Japan, Latin America, and the United States (U.S.). The data reflect 7,079,145 total ART cycles utilizing both fresh and previously cryopreserved embryos from autologous oocytes that resulted in 1,454,724 live births. This review focused on the following measures: ART cycle volume, use of cryopreserved embryos, single embryo transfer (SET), live birth rates in fresh and frozen-thawed cycles, and perinatal outcomes in recent years. Results SETs and utilization of frozen-thawed embryos increased worldwide over the study period. In 2012 SET utilization in all ART cycles was highest in Japan and Australia/New Zealand (82.6% and 76.3% respectively) and lowest in Latin America (16.0%). While gradual improvements in live birth rates were observed in most regions, some demonstrated declines. By 2012–2013, fresh cycle live birth rates were highest in the U.S. (29%) and lowest in Japan (5%). In Japan, the observed decline in fresh cycle live birth rate coincided with transition to minimal stimulation protocols, transfer of frozen-thawed rather than fresh embryos, and implementation of an SET policy. Similarly, implementation of an SET policy in parts of Canada was followed by a decline in fresh cycle live birth rate. Increasing live birth rates in frozen-thawed embryo cycles, seen all over the world, partially compensated for declines in fresh ART cycles. During 2012–2013 Australia/New Zealand and Japan reported the lowest multiple delivery rates of 5.6 and 4% respectively while the US had the highest of 27%. In recent years, preterm delivery rates in all regions ranged between 9.0 to 16.6% for singletons, 53.9 to 67.3% for twins, and 91.4 to 100% for triplets and higher order multiples. Inconsistencies in the way perinatal outcome data are presented by various registries, made comparison between regions difficult. Conclusions ART practices are characterized by outcome differences between regions. International consensus on the definition of ART success, which accounts for perinatal outcomes, may help to standardize worldwide ART practice and improve outcomes. Trial registration PROSPERO ( CRD42016033011 )

Air pollution is a cause of concern for human health. For instance, it is associated with an increased risk for cancer, cardiovascular and respiratory disorders. In vitro and in vivo studies suggested that air pollutants could act as endocrine disruptors, promote oxidative stress and exert genotoxic effect. Whether air pollution affects female infertility is under debate. The aim of the present study was to conduct a systematic review of studies that evaluated the impact of air pollution on female infertility. We systematically searched the MEDLINE (PubMed) and SCOPUS databases to identify all relevant studies published before October 2017. No time or language restrictions were adopted, and queries were limited to human studies. We also hand-searched the reference lists of relevant studies to ensure we did not miss pertinent studies. The risk of bias and quality assessment of the studies identified were performed using the Newcastle-Ottawa Scale. Primary outcomes were conception rate after spontaneous intercourse and live birth rate after in vitro fertilization (IVF) procedures. Secondary outcomes were first trimester miscarriage, stillbirths, infertility, number of oocytes and embryo retrieved. Eleven articles were included in the analysis. We found that in the IVF population, nitrogen dioxide and ozone were associated with a reduced live birth rate while particulate matter of 10 mm was associated with increased miscarriage. Furthermore, in the general population, particulate matter of 2.5 mm and between 2.5 and 10 mm were associated with reduced fecundability, whereas sulfur dioxide , carbon monoxide and nitrogen dioxide might promote miscarriage and stillbirths. The main limitation of our findigns resides in the fact that the desegn of studies included are observational and retrospective. Furthermore, there was a wide heterogenity among studies. Although larger trials are required before drawing definitive conclusions, it seems that air pollution could represent a matter of concern for female infertility.