Male neonatal sex was associated with several adverse perinatal outcomes, including acidemia and neurological complications, in a Swedish cohort of 87,840 singleton vaginal births.
Male neonates had higher adjusted odds of umbilical cord arterial pH below 7.05 (neonatal acidemia) than female neonates.
Male neonates had higher adjusted odds of a low 5-minute Apgar score (<7).
Male neonates had higher adjusted odds of needing CPAP and of NICU admission.
Male neonates had higher adjusted odds of hypoxic-ischemic encephalopathy and central nervous system disease.
The study used a large retrospective multicenter Swedish register cohort of live-born, singleton, vaginal births, of which 51.6% were male.
The authors conclude that male sex is associated with multiple adverse perinatal outcomes and that the findings warrant further investigation.
This study looked at records of nearly 88,000 babies born vaginally in southern Sweden between 1995 and 2015. It compared boys and girls on measures of how well they did at and soon after birth, including the acidity of blood from the umbilical cord. Low cord blood pH (below 7.05) is used in Sweden to flag acidemia, a sign the baby may have had too little oxygen around birth. After accounting for the mother's age, the baby's gestational age, and birth weight, boys were more likely than girls to have low cord pH, low Apgar scores at 5 minutes, and a need for breathing support (CPAP) or neonatal intensive care. They were also more likely to be diagnosed with hypoxic-ischemic encephalopathy (brain injury from low oxygen) and other central nervous system diseases. The increases in odds ranged from about 18% for low cord pH to more than double for hypoxic-ischemic encephalopathy. This research suggests boys may be biologically more vulnerable around birth, possibly because of differences in the placenta or in inflammatory responses, though this study did not test those explanations. Because it is an observational, retrospective study, it shows associations rather than causes, and the authors say further investigation is warranted.
Akel W, Abdulilah S, Zaigham M. (2026). Sex-specific adverse neonatal outcomes and umbilical cord arterial ph at birth: a population-based cohort study.. BMC pediatrics. https://doi.org/10.1186/s12887-026-07865-5