We have heard for years that smoking is hazardous to your health, but this is the first study that shows it may also determine the sex of your baby. The study in the Lancet says researchers followed more than 11,000 couples from the time of conception. The lowest ratio of boys was found in couples where both mother and father were heavy smokers, which the study describes as more than 20 cigarettes a day. The chance of having a boy was also reduced among couples where just the dad was a heavy smoker, and mom didn't smoke at all. Researchers hypothesize that sperm cells carrying the Y-chromosome, which is responsible for baby boys, are more sensitive to unfavorable changes caused by smoking.
Researchers say their findings support the thinking that chronic exposure to toxins like cigarette smoke have contributed to decrease in the number of boys born over the past few decades. They say the male chromosome and reproductive system are especially sensitive to such environmental toxins.
Researchers say they recorded a couple's smoking habits for the three months prior to conception. The study is published in the journal Lancet and was conducted by researchers in Denmark and Japan. Dr. Claus Yding Anderson, at the Laboratory of Reproductive Biology at the University Hospital of Copenhagen, Denmark, teamed up with Dr. Misao Fukuda at the Fukuda Ladies Clinic in Hyogo, Japan.
Anne Grete Byskov, one of the Danish investigators who worked with Dr. Anderson, comments: "Our working hypothesis is that the sperm cells carrying the Y-chromosome, which are responsible for male children, and, are more sensitive to unfavorable changes caused by smoking than sperm cells with an X-chromosome. Such affected Y-sperm cells might be less prone to fertilize and/or produce less viable embryos.
Smoking may cause a stress effect on the sperm cell itself since the sex ratio also declined when the mother smoked but not the father." This quote comes by e-mail, but was not published in the Lancet report.