April 18, 2002 -- A smoke after sex may not be such a good idea -- for more reasons than lung cancer. A new study from Denmark shows that couples who smoke while they're trying to conceive could reduce their chances of having male offspring.
The number of male babies has declined over the past few decades in a number of countries, including the U.S., Canada, Denmark, England, Wales, the Netherlands, Germany, Sweden, Norway, and Finland, writes study author Anne Grete Byskov, PhD, a professor at the University Hospital of Copenhagen.
The reason for this decline has not been clear, but scientists have speculated that chronic exposure to toxic environmental agents that affect the male reproductive system could be the cause. In two separate environmental disasters, dioxin and methylmercury contamination have been linked to fewer male babies. However, stress has also been cited as a cause.
In their study, Byskov and colleagues analyzed birth records of 11,815 infants. They also questioned each mother about her and her spouse's daily smoking habits as much as three months before conception was confirmed.
Their findings? The number of male babies declined with increasing numbers of cigarettes smoked by mothers and fathers. The male-to-female sex ratio was 1.21 among parents who never smoked, whereas the lowest male-to-female ratio was 0.82 among parents who both smoked more than 20 cigarettes a day. Even if the mother didn't smoke, a father who smoked more than 20 a day reduced the odds to 0.98.
"Sperm cells carrying the Y-chromosome -- responsible for male children -- are more sensitive to unfavorable changes caused by smoking than sperm cells with an X-chromosome," Byskov says in a news release.
"Such affected Y-sperm cells might be less prone to fertilize and/or produce less viable embryos," she says. "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."