The Risk of Perinatal Mortality in Nulliparous Women Compared to Primiparous Women at Term

Objective The aim of this study was to determine the rate of perinatal mortality among nulliparous women compared with primiparous women at term and further characterize the risk of stillbirth by each week of gestation. Study Design This is a retrospective cohort study of all term, singleton, nonanomalous births comparing perinatal mortality (stillbirth and neonatal death [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 with parity, adjusting for maternal age, race, body mass index, pregestational 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 with primiparous women. The risk of stillbirth in nulliparous women was greater at every gestational age between 37 ^0/7and 41 ^0/7weeks compared with primiparous women. Nulliparous women also had increased odds of small for gestational age infants at less than 10% birth weight (aOR, 1.76; 95% CI, 1.72-1.79), less than 5% birth weight (aOR, 1.91; 95% CI, 1.86-1.98), and less than 3% birth weight (aOR, 2.02; 95% CI, 1.93-2.11). Conclusion Perinatal mortality is significantly greater in nulliparous women compared with 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. Key Points Parity is associated with perinatal mortality. Perinatal mortality is significantly greater in nulliparous women compared with primiparous women. The risk of stillbirth in nulliparous women is greater at every gestational age compared with primiparous women.