Sex Prediction Based on Previous Children's Sex
No, having three girls does not make a woman biologically more likely to have a fourth girl—the sex of previous children does not influence the biological probability of the next child's sex. The observed clustering of same-sex siblings in families is primarily explained by parental behavior (continuing to have children until achieving the desired sex mix) rather than any biological predisposition 1.
The Biological Reality
Each pregnancy has independent sex determination with a baseline male-to-female ratio of approximately 1.055 (slightly favoring boys), regardless of previous children's sex 2.
Large-scale Norwegian registry data covering over 540,000 women with multiple births demonstrated that the sex composition of already-born siblings had no influence on the sex of the next child 1.
When analyzing families with two or more previous children of the same sex, no biological mechanism was identified that would predispose toward continuing the same-sex pattern 1.
Why Same-Sex Families Appear Common
Behavioral factors, not biological ones, explain same-sex sibling clusters. Women with children of only one sex (whether all boys or all girls) have a 14-15% higher probability of having additional children compared to women with mixed-sex children 1.
This increased fertility behavior is highest among mothers with boys only, suggesting parents continue attempting to have a child of the opposite sex 1.
The apparent excess of same-sex families in the population reflects family planning decisions rather than biological predisposition 1.
What Actually Influences Sex Ratio
Parity and multiple births affect the probability of having a boy versus girl, but previous siblings' sex composition does not 1.
Some research suggests within-sibship variation (Poisson variation) and possibly between-sibship variation (Lexis variation) may exist, meaning individual families might have slightly different baseline probabilities, though this remains controversial 3.
Japanese cohort data (n=62,718) found that when there were more than two children previously, the male/female sex ratio was significantly higher among boy-only mothers than girl-only mothers, but this likely reflects selection bias from families continuing to pursue a specific sex rather than biological predisposition 2.
Clinical Counseling Points
Reassure parents that each pregnancy is an independent event with approximately 51-52% chance of a boy and 48-49% chance of a girl 2.
The common misconception that "we're due for a boy/girl" represents the gambler's fallacy—each conception has the same baseline probability regardless of previous outcomes 3.
If parents express strong sex preference, acknowledge that continuing pregnancies to achieve desired sex composition is a common behavioral pattern but does not reflect biological likelihood 1.