Offspring sex and pregnancy outcome by length of gestation.

BACKGROUND: It takes a higher number of male than female embryos to produce a live born infant. The unbalanced pregnancy survival by offspring sex may also be reflected in higher proportion of preterm male births, and in unbalanced sex distribution in certain pregnancy conditions, such as preeclampsia.
METHODS: We used data from the Medical Birth Registry of Norway, a population-based registry that has recorded births since 1967. For this study, we used information on offspring sex and length of gestation that was available for 1691053 (92.8%) singleton births among a total of 1822982 births from 1967 to 1998. We estimated sex ratios and perinatal mortality by length of gestation, and assessed whether the ratio of offspring sex in preeclampsia varied by length of gestation.
RESULTS: For preterm births, there was a strong male dominance. Within five categories of gestational age between 16 and 36 weeks, the male/female ratios were 2.48, 1.26, 1.28, 1.32, and 1.28. At weeks 37-39, the sex ratio was 1.17, but at weeks 40-42 the number of male and female births was practically identical (sex ratio 1.00). Over all, the male/female ratio was 1.06. Perinatal mortality was consistently higher in males across the whole range of gestational age; in total it was 21% (95% CI, 18-25%) higher in male offspring. In preeclampsia with preterm delivery (<37 weeks), the sex ratio was reversed: female offspring was substantially more common than males (sex ratio 0.87), but in preeclampsia with delivery at term (37-42 weeks), the proportion of males was higher (sex ratio 1.06) than for females.
CONCLUSION: The sex differences by length of gestation and in preeclampsia may reflect that male embryos are subject to stronger intrauterine selection forces than females. Possibly, implantation may be the critical event, where offspring sex may be one of the factors that determine success.