Women whose pregnancies seem to be going on forever now know who to blame - the father.
Longer pregnancies could be down to dads
Researchers suggest a father's genes appear to play a major part in deciding the timing of the birth, and the chance of having a 'prolonged pregnancies'.
Pregnancies are said to be prolonged if they are longer than 42 weeks, or 294 days. The condition affects around 5% of pregnancies.
It is linked to a higher risk of complications for both mother and child, but little is known about its causes.
Danish researchers found women who had experienced one prolonged pregnancy were more likely to experience another if they were with the same partner.
This result suggests that the timing of birth may, in part, be determined by parental genes
Annette Wind Olsen, Science Centre University in Aarhus
Those with new partners had a lower risk.
The researchers say their findings suggest paternal genes may therefore affect the timing of the birth.
Doctors from the Science Centre University in Aarhus looked at data for births between 1980 and 1994.
They identified 21,000 women whose first pregnancy had lasted longer than 42 weeks between. All had had a second baby.
Another 7,000 women whose first pregnancy had lasted between 37 and 41 weeks were also studied.
Women who had had one prolonged pregnancy had a 20% risk of having a second one.
If the first pregnancy had lasted for 44 weeks, the risk increased to 30%.
But the risk of a second prolonged pregnancy was reduced to 15% if the woman had changed partners between pregnancies.
The length of pregnancy differed by more than a week in women who had changed partners, compared with those who had not.
In the group of women who had a 'normal' length pregnancy, only 7.7% had a prolonged second pregnancy.
Writing in the British Medical Journal, the researchers led by Annette Wind Olsen, said: "Women had a reduced risk of recurrent post-term delivery if they changed partner between pregnancies.
"We believe this is a new observation. This result suggests that the timing of birth may, in part, be determined by parental genes."
But UK experts say other factors could have influenced the study's findings.
Professor Andrew Calder, a consultant obstetrician at the University of Edinburgh, told BBC News Online: "It may well be that there is a genetic aspect."
He added that factors such as pre-eclampsia could affect pregnancy length, and therefore skew the findings.
"We know that pre-eclampsia is commoner in the first pregnancy with a particular partner."
"If a woman has changed partners and then got pre-eclampsia, that would distort the length of pregnancy in that group.
He added: "The research also had no information about how many women were induced before they reached 42 weeks."