The risk of perinatal mortality in nulliparous women compared to primiparous women at term.

Objective: To determine the rate of perinatal mortality among nulliparous women compared to primiparous women at term and further characterize the risk of stillbirth by each week of gestation. Study Design: A retrospective cohort study of all term, singleton, non-anomalous births comparing perinatal mortality (stillbirth and neonatal demise [NND]) between primiparous (parity=1, with no history of abortion) and nulliparous (parity = 0) women who delivered in California between 2007 and 2011. Chi squared tests and multivariable logistic regression analyses were performed to determine the frequencies and strength of association of perinatal mortality and parity, adjusting for maternal age, race, body mass index (BMI), pre-gestational diabetes, chronic hypertension, fetal sex, smoking status, and socioeconomic status. The risk of stillbirth at each gestational age at term was calculated using a pregnancies-at-risk life table method. A p-value less than 0.05 was used to indicate statistical significance. Results: Of 1,317,761 total deliveries, 765,995 (58.1%) were to nulliparous women and 551,766 (41.9%) were to primiparous women with one prior birth. Nulliparous women had increased odds of stillbirth (adjusted odds ratio [aOR] 3.30, 95% confidence interval (CI) 2.93-3.72) and NND (aOR 1.54, 95% CI 1.19-1.98) compared to primiparous women. The risk of stillbirth in nulliparous women was greater at every gestational age between 37 0/7 and 41 0/7 weeks compared to primiparous women. Nulliparous women also had increased odds of small for gestational age (SGA) infants at <10% birth weight (aOR 1.76, 95% CI 1.72-1.79), <5% birth weight (aOR 1.91, 95% CI 1.86-1.98), and <3% birth weight (aOR 2.02, 95% CI 1.93-2.11). Conclusion: Perinatal mortality is significantly greater in nulliparous women compared to primiparous women with term deliveries. These findings suggest that low-risk nulliparous women may require increased surveillance. There may be a role in improving maternal health by maximizing physiologic adaptation in nulliparous women.