Overview

• In families with three boys the chance of a fourth son rises to 61 percent, and in those with three girls the odds of another daughter reach 58 percent.
• The study, drawing on 58,007 U.S. nurses' records from 1956 to 2015, controlled for stopping rules by excluding each woman's final birth from key analyses.
• Mothers aged 29 or older at first birth showed a 13 percent higher likelihood of having only boys or only girls, a trend researchers link to age-related shifts in vaginal pH and follicular-phase duration.
• Genomic analysis identified a variant in NSUN6 associated with all-female offspring and a nearby TSHZ1 variant correlated with all-male offspring.
• Critics highlight the study's predominantly white, American sample and lack of paternal data and urge replication in other populations to validate these findings.