Background The work patterns of pregnant women may be related to adverse obstetric and perinatal outcomes. This study aimed to clarify the effects of weekly working time according to frequencies of night shifts during pregnancy on adverse outcomes in Japan. Methods The Japan Environment and Children's Study, a prospective cohort study, was conducted in 15 regions nationwide in Japan. The study population included pregnant women with singleton pregnancies (n = 99 744). The mothers’ working hours and frequencies of night shifts during the first and the second/third trimesters were assessed using a self‐administered questionnaire. Outcome data were collected from medical transcripts. Results Compared with nonworking women, women who worked during pregnancy had significantly increased adjusted odds ratios (aORs) of threatened miscarriage (maximum aOR: 1.47, 95% confidence interval [95% CI]: 1.26‐1.73) and of threatened preterm labor (maximum aOR: 1.63, 95% CI: 1.41‐1.87). Increased aORs were observed for hypertensive disorders of pregnancy (maximum aOR: 2.02, 95% CI: 1.39‐2.93) in women working ≥36 hours per week with night shifts, for vacuum/forceps delivery (maximum aOR: 1.34, 95% CI: 1.22‐1.48) at ≥36 hours with or without night shifts, and for small‐for‐gestational‐age babies (aOR: 1.32, 95% CI: 1.10‐1.59) at ≥46 hours with night shifts. In contrast, lower aORs were observed for gestational diabetes and meconium‐stained amniotic fluid in women working without night shifts. Conclusions Work during pregnancy slightly increased the risks of threatened miscarriage and threatened preterm labor. Long working hours increased the risks of hypertensive disorders of pregnancy, vacuum/forceps delivery, and small‐for‐gestational‐age babies.

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Problem Several studies have reported the increased risk of preterm birth, premature rupture of membranes, and low birth weight in patients with recurrent pregnancy loss (RPL). There have been a limited number of large population‐based studies examining adverse pregnancy and perinatal outcome after RPL. Multiple‐imputed analyses (MIA) adjusting for biases due to missing data is also lacking. Method of study A nationwide birth cohort study known as the “Japan Environment and Children’s Study (JECS)” was conducted by the Ministry of the Environment. The subjects consisted of 104 102 registered children (including fetuses or embryos). Results No increased risk of a congenital anomaly, aneuploidy, neonatal asphyxia, or a small for date infant was observed among the children from women with a history of RPL. A novel increased risk of placental adhesion and uterine infection was found. The adjusted ORs using MIA in women with three or more PL were 1.76 (95% CI, 1.04‐2.96) for a stillbirth, 1.68 (1.12‐2.52) for a pregnancy loss, 2.53 (1.17‐5.47) for placental adhesion, 1.87 (1.37‐2.55) and 1.60 (.99‐2.57) for mild and severe hypertensive disorders of pregnancy, respectively, 1.94 (1.06‐3.55) for uterine infection, 1.28 (1.11‐1.47) for caesarean section and .86 (.76‐.98) for a male infant. Conclusion MIA better quantified the risk, which could encourage women who might hesitate to attempt a subsequent pregnancy.

Background Since venous thromboembolism (VTE) is one of the causes of maternal mortality, several guidelines recommend prophylaxis using low molecular weight heparin for women in high-risk groups. The number of large population-based studies examining predictors for VTE has been limited, and there has been no study based on a Japanese population. Objective Our objective was to examine VTE risk factor during the pregnancy and post-partum period. Materials and Methods A nationwide birth cohort study known as the ‘Japan Environment and Children's Study (JECS)’ was conducted by the Ministry of the Environment. The subjects consisted of 103,070 pregnancies recruited by the JECS between January 2011 and March 2014. Pregnant women completed the questionnaires during the first and second/third trimester. Their medical records were transcribed by physicians or research coordinators at registration, just after delivery and at 1 month after delivery. Results The frequency of VTE was 7.5 per 10,000 pregnancies (77 of 103,070) during the pregnancy and post-partum period. After the adjustment of multiple covariates for each factor, endometriosis and recurrent pregnancy loss (RPL) were identified as novel independent risk factors for VTE. Adjusted odds ratios were as follows: 2.70 (95% confidence interval, 1.21–6.00) for endometriosis and 6.13 (2.48–15.16) for RPL. Threatened abortion, threatened pre-term birth, pre-term birth and caesarean section were ascertained to be risk factors for VTE. Conclusion Careful attention should be given to novel predictors, such as endometriosis and a history of RPL, to prevent VTE during the pregnancy and post-partum period.