Risks of preterm birth and growth restriction in second births after a first-born male infant

A first-born male infant may lead to an exaggerated immune response in the mother in subsequent pregnancies, increasing the risk of preterm birth (PTB) and growth restriction. This study investigated whether risks of PTB and growth restriction are greater among infants preceded by a first-born male infant and evaluated if the associations differ by paternity change or the second-born's sex. We conducted a population-based cohort study using 2003–2014 Washington State birth certificate data and included mothers and birth year frequency-matched second live-born infants preceded by a first-born male (n = 58,704) or by a first-born female infant (n = 58,704). Using stratified analyses, we estimated adjusted relative risks (RRs) of PTB, low birthweight (LBW), and small for gestational age (SGA). Second-born infants preceded by a first-born male had greater risks of PTB (RR = 1.14; 95% CI: 1.09–1.19), LBW (RR = 1.17; 1.10–1.24), and SGA (RR = 1.13; 1.08–1.18). The RR was elevated for indicated PTB (RR = 1.19; 1.10–1.29), preterm premature rupture of membranes (RR = 1.15; 1.01–1.32), and spontaneous PTB (RR = 1.12; 1.05–1.20). Associations did not differ by second-born infant's sex or paternity change. Having a first-born male infant was associated with a greater risk of PTB, LBW, and SGA in the second-born infant.