Related Experiment Video
Updated: Aug 14, 2026

Anogenital Distance and Perineal Measurements of the Pelvic Organ Prolapse (POP) Quantification System
Published on: September 20, 2018
[Why do some women only give birth to boys or to girls?]
Tonje Lippert1, Rolv Skjaerven, Kjell A Salvesen
1Det medisinske fakultet, Norges teknisk-naturvitenskapelige universitet, 7489 Trondheim. lippert@stud.ntnu.no
Background:
There are more families with many children of only one sex than can be explained by unconditional probability.
Material And Methods:
Data from the Medical Birth Registry of Norway, covering more than 540,000 women with two, three and four births for the period 1967-2003, were used to study how the sex distributions of children already born affected the probability of a new birth and the sex of the next sibling.
Results:
Women with two children of the same sex had a higher probability of having more children compared to women with two children of both sexes (RR=1.14, (1.14-1.15)). This also applied to mothers with three children (RR=1.15 (1.13-1.17)). The probability was highest for mothers with boys only. Multiple births and parity affected the probability of giving birth to a boy versus a girl, but the sex composition of already born siblings had no influence.
Interpretation:
We have no evidence that some parents may have a probability of having a boy versus a girl that differs from the rest of the population. The fact that there are more siblings of only one sex is mainly behavioural; some mothers with only girls or only boys keep having more children in an attempt to receive a child of the opposite sex.
Related Concept Videos
X and Y Chromosomes
The germline cells such as egg and sperm cells carry only half the number of chromosomes, i.e., 22 autosomes and one sex chromosome. All eggs have an X chromosome, while sperm cells can carry an X or...
Sex-linked Disorders
The Y Chromosome Determines Maleness
Evolution
Around 300 million years ago, the two sex chromosomes diverged from two identical autosomal chromosomes. Over time, the Y chromosome has lost most of its genes, shrinking in size. Today,...
X-linked Traits
X-linked Traits
The Ratio of X Chromosome to Autosomes
Normal male Drosophila has a ratio of one X chromosome to two sets of autosomes. In contrast, normal female Drosophila...
