Objectives: This article derives new mathematic formulas to calculate the probability that the husband of a D-negative woman is heterozygous for the D antigen.
Study design: Current published formulas and tables that estimate this probability are biased in that they incorrectly assume that the father is randomly selected from the general population. With Bayes' theorem, new formulas were derived to eliminate this bias.
Results: The probability of the husband being homozygous or heterozygous for the D antigen can be expressed as a function of the number of previous D-positive children. If he has any D-negative children, he is an obligate heterozygote.
Conclusion: Previously published formulas and tables that estimate the likelihood of the father being heterozygous or homozygous at the D locus should no longer be used in the obstetric setting.