THURSDAY, Feb. 27, 2003 (HealthDayNews) -- You can blame mom for just about everything, but not for a longer-than-normal pregnancy.
It's the father's genes, not the mother's, that help determine the timing of the birth, suggest researchers in the March 1 issue of the British Medical Journal.
"I looked into all pregnancy lengths and the pregnancy length differed if the woman changed partners," says Dr. Annette Wind Olesen, co-author of the study. "There's an effect of paternal genes on length of gestation."
Other experts wonder how useful this information is, at least for women who have access to today's sophisticated technology and close monitoring.
"This is clinically insignificant information for me," says Dr. Ralph Dauterive, chief of obstetrics and gynecology at the Ochsner Clinic Foundation in New Orleans. "Our ability to monitor and induce pregnancies has advanced to the point that no pregnancy should reach a point of increased risk."
Prolonged pregnancies, defined as lasting more than 42 weeks or 294 days since the first day of the last period, occur 5 percent of the time and carry a higher risk of complications for both the mother and child, the study authors say.
Most women deliver between 38 and 42 weeks, according to the American Academy of Family Physicians, while a normal pregnancy lasts 40 weeks. Dauterive will not let any pregnancy progress past the 42nd week. If the baby gives no sign of emerging, labor is induced sometime during the 41st week.
This study analyzed data recorded in the Danish medical birth registry on women who had had prolonged pregnancies for their first delivery and a subsequent delivery between 1980 and 1994. They also looked at the first two pregnancies in a 5 percent sample of all women who had pregnancies recorded between 1980 and 1992. There were 21,746 sibling pairs in the overdue group and 7,009 sibling pairs in the on-time group.
"We adjusted for maternal age, how long between pregnancies and the social status of the mother and also the region," Olesen says.
Overall, if a woman had one later-than-expected delivery, the risk of it happening again was 19.9 percent.
When the children had different fathers, however, the risk of recurrence went down to 15.4 percent.
The length of pregnancy for women who had changed partners differed by more than one week, compared with those who stayed with the same man.
Paternal genes, therefore, seem to be the culprit.
Olesen believes the information will let health-care professionals concentrate resources on high-risk women. "We know very little about prolonged pregnancies, who they are," she says. "If we have to concentrate on surveillance of those who are at risk of poor outcomes, the first step is to find out which factors are associated with this."
Also, little is known about the factors contributing to delivery. "We think this may contribute to the very little knowledge we have about the triggers of human delivery," Olesen adds.
There's a parallel in another area of pregnancy, says Marie-Louise Lubs, an associate professor of pediatric genetics at the University of Miami School of Medicine. "The risk for preeclampsia [a potentially life-threatening complication of pregnancy] is about 15 percent during the first pregnancy," she says. "For the second child, it's less than 5 percent." If you change partners, however, the risk during the second pregnancy reverts to 15 percent, which is exactly the same cycle seen in this Danish study. Lubs says it apparently has to do with the woman's ability to adjust to antigens that come from her husband or partner.
However, for women who go beyond the due date, the clinical significance is less clear.
"What if you do know that this person has an increased risk of having a pregnancy go beyond 42 weeks. Do you tell her, 'Don't get pregnant with that man again?' No, that's ridiculous," Dauterive says. "When you get to 41 weeks, you recommend induction and you already knew that anyway."
The data suggest that real problems don't begin until after the 43rd week, Dauterive adds. For a long time, physicians would test weekly or biweekly starting at 42 weeks and, if everything checked out, would let the pregnancy progress to the 43rd week.
Now, however, there seems to be a trend towards earlier and earlier induced births. "There's an alarming trend in the United States of inducing patients electively after the 39th week within a week of their due date," Dauterive says. "It's a convenience issue. You can pick the day and you can do it during the day. People are pushing the envelope."
In this case, you might say the delivery date is determined by the mother or the doctor -- dad may not factor into the equation.
More information
For more on post-term pregnancy, visit the Children's Hospital of The King's Daughters.
The American College of Obstetricians and Gynecologists has information on inducing labor.
