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Patterns: If It Takes a Long Time, It May Be a Boy

The longer a woman takes to get pregnant, the greater her chance of having a boy, new research says.

An investigation of 5,283 Dutch women revealed that 57.6 percent of those who took longer than 12 months to become pregnant had boys, compared with 51.1 percent boys among those who became pregnant in less than a year.

Although human semen has equal numbers of Y-bearing male sperm and X-bearing female sperm, the males may be better swimmers in viscous cervical mucus, the authors speculate. That mucus makes impregnation more difficult.

"The best-supported hypothesis," said Dr. Luc J. M. Smits, the lead author, "is that the Y chromosome is lighter than the X chromosome, while Y- and X-bearing sperms have the same locomotive power. In liquid fluids, this difference would not cause differential speed, but in viscous fluids it would." Dr. Smits is a lecturer in epidemiology at the University of Maastricht in the Netherlands.

The time span taken to become pregnant was established by asking women after delivery how many months of unprotected intercourse they experienced before the pregnancy.

Seventeen percent of the pregnancies were unplanned, but excluding these from the calculations did not change the results. In couples who had received medical help in getting pregnant, time to pregnancy made no difference in the sex of the offspring.

The study, which appeared in the Dec. 17 issue of BMJ, the British medical journal, found that maternal age, body mass index and smoking or alcohol use made no difference in the findings. The authors calculated that for each additional year of trying to get pregnant, a couple had a 4 percent increased probability of having a boy.

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