Experts say breech births can be handled safely
Some babies are born bottom-first because of genetic traits inherited from either their mother or father, Norwegian researchers have said.
Fewer than one in 20 is delivered this way up, but a natural breech birth carries extra risks to the child.
A study of 387,000 births, in the British Medical Journal, found a baby had double the chance of being breech if their mother or father was too.
But midwives said parents should not worry too much.
As many as one in four babies are in the wrong position at the mid-point of pregnancy, but all but 3% or 4% are head-down by the time they are delivered.
The precise reasons why a baby might be in the breech position are not known, although the anatomy of the mother - in particular the shape of her womb - can play a strong role.
Babies are designed to be born head-first, and coming out the other way round increases the chance of breathing problems at the moment of delivery.
For this reason, many women with full-term breech babies opt for caesarean sections.
The researchers from the University of Bergen looked at the records of more than 387,000 parents and their first-born children born between 1967 and 2004.
They found an identical increase in risk passed from both male and female parents born in breech position.
While a mother might be able to pass on the increased risk through inherited differences in her physical makeup, any risk passed from a father raises the possibility of a genetic trait carried by the baby rather than the mother.
However, other specialists say the picture is less clear.
Professor Janet Hardy, from the University of Massachusetts Medical School, said that there could be a separate, undetected factor that was increasing the chance of a breech birth in these families.
She said: "Clinicians should continue to gather information during early prenatal care on maternal and paternal birth presentation and other potential risk factors for breech delivery."
Mervi Jokinen, from the Royal College of Midwives, said the findings were "intriguing".
"We always tend to ask mothers if they know how they were delivered, and some midwives will ask the partners as well, just to record this in the notes.
"But on the whole, women should not be too concerned about the possibility of a breech baby, as long as she is receiving proper ante-natal care."
Henry Annan, a spokesperson for the Royal College of Obstetricians and Gynaecologists, said that having a breech delivery approximately doubled the risks of complications to the baby.
"Having a breech baby does increase the dangers, although, with proper management, the chances are that the baby will be born healthy.
"I think a lot of parents will be unaware of whether they were born breech or not, but this is still an interesting study."