Heightened risk of preterm birth and growth restriction after a first-born son.
OBJECTIVE
In Scandinavia, delivery of a first-born son elevates the risk of preterm delivery and intrauterine growth restriction of the next-born infant. External validity of these results remains unclear. We test this hypothesis for preterm delivery and growth restriction using the linked California birth cohort file. We examined the hypothesis separately by race and/or ethnicity.


METHODS
We retrieved data on 2,852,976 births to 1,426,488 mothers with at least two live births. Our within-mother tests applied Cox proportional hazards (preterm delivery, defined as less than 37 weeks gestation) and linear regression models (birth weight for gestational age percentiles).


RESULTS
For non-Hispanic whites, Hispanics, Asians, and American Indian and/or Alaska Natives, analyses indicate heightened risk of preterm delivery and growth restriction after a first-born male. The race-specific hazard ratios for preterm delivery range from 1.07 to 1.18. Regression coefficients for birth weight for gestational age percentile range from -0.73 to -1.49. The 95% confidence intervals for all these estimates do not contain the null. By contrast, we could not reject the null for non-Hispanic black mothers.


CONCLUSIONS
Whereas California findings generally support those from Scandinavia, the null results among non-Hispanic black mothers suggest that we do not detect adverse outcomes after a first-born male in all racial and/or ethnic groups.
Public Health & Planning, Policy, and Design, University of California, Irvine, CA. Electronic address: tim.bruckner@uci.edu.
Tim A
TA
Bruckner
Division of Neonatal and Developmental Medicine, Department of Pediatrics, Stanford University School of Medicine, Stanford, CA.
Jonathan A
JA
Mayo
Division of Neonatal and Developmental Medicine, Department of Pediatrics, Stanford University School of Medicine, Stanford, CA.
Jeffrey B
JB
Gould
Division of Neonatal and Developmental Medicine, Department of Pediatrics, Stanford University School of Medicine, Stanford, CA.
David K
DK
Stevenson
Division of Allergy, Immunology and Rheumatology, Department of Pediatrics, Stanford University School of Medicine, Stanford, CA.
David B
DB
Lewis
Division of Neonatal and Developmental Medicine, Department of Pediatrics, Stanford University School of Medicine, Stanford, CA.
Gary M
GM
Shaw
Division of Neonatal and Developmental Medicine, Department of Pediatrics, Stanford University School of Medicine, Stanford, CA.
Suzan L
SL
Carmichael
D001741:Black or African American / Q000706:statistics & numerical data; D001722:Birth Order; D001724:Birth Weight; D002140:California / Q000453:epidemiology; D005006:Ethnicity / Q000706:statistics & numerical data*; D005191:Family Characteristics; D005260:Female; D005865:Gestational Age; D006630:Hispanic or Latino / Q000706:statistics & numerical data; D006801:Humans; D007198:Indians, North American / Q000706:statistics & numerical data; D007231:Infant, Newborn; D016014:Linear Models; D008297:Male; D011247:Pregnancy; D011256:Pregnancy Outcome / Q000208:ethnology; D047928:Premature Birth / Q000208:ethnology*; D016016:Proportional Hazards Models; D012737:Sex Factors; D035781:Siblings; D044465:White People / Q000706:statistics & numerical data
Birth order; First-born; Growth restriction; Male; Preterm; Sibling