When pregnant, most of us will at some point wonder how big our baby will be. The notion of getting a seven and a half pound baby through the birth canal seems nonsensical. So imagine how it must feel to be told that your baby is clinically oversized (that is, over 8lb 8oz - foetal macrosomia is the technical term).
But most estimates are inaccurate. Ultrasound technology can be up to a couple of pounds out and manual palpitation is unreliable. So perhaps it is good news that researchers at Duke University Medical Center in Durham, North Carolina, have come up with an equation that predicts birth weight to within an average of 8% (ultrasound accuracy varies at best by about 10% - often a difference of 1lb or more).
Led by Dr Gerard Nahum, the team studied 244 healthy Caucasian women giving birth to full-term single babies. They used factors known to affect foetal size, such as sex, the mother's height, starting weight and weight gain in the third trimester, plus the father's height. The detail is impressive: even altitude, Nahum says, can influence the size of a baby (people who live up mountains have, on average, smaller babies).
Currently the equation only works for Caucasian women (the team are working on other ethnic groups now), but should it become the clinical norm, its main advantage is that it can map the baby's growth throughout the final trimester. This, says Nahum, would enable women expecting large babies to plan an early induction or caesarean.
There are clinical risks, Nahum explains, associated with giving birth vaginally to a big baby. The baby can (though rarely) injure its brachial plexus, a condensation of nerves in the neck and shoulder which can lead to palsies in one arm; and arm and shoulder bones can fracture . For the mother, an oversized baby can cause bad tearing, incontinence (the pelvic floor takes a battering) and possibly prolapse (the womb or vagina sags downwards, usually later in life). There is also a risk of shoulder distocia during the birth - the baby's shoulder gets stuck on the perineum. For these reasons, Nahum believes a scheduled caesarean or induction for any baby over 8lb 8oz is advisable.
This, of course, is controversial in an age where one in five babies is born surgically, and intervention is the norm. You will hear people argue that we are producing bigger babies and our pelvises just can't cope with these over-nourished progeny but, says Nahum, there is no evidence for this. The average girl is still 7lbs, the average boy 7lb 10oz. So why interfere?
James Walker, professor of obstetrics and gynaecology at the University of Leeds medical school says there is usually no reason to. "Plenty of women give birth to 10lb babies with absolutely no complications. "The question should not be, 'Is the baby big?' but 'Is the baby big for the mother?'" He quotes a rather unflattering medical adage here: "Shetland ponies," he says, "have Shetland ponies." Problems can arise, however, if "the baby is out of ratio with the mother" (think 5ft woman, 10lb baby).
So will the equation help women? Jo, who gave birth to Ollie, who weighed in at 9lb 11oz four years ago, and is six months pregnant with her second, thinks it will: "My last birth and postpartum were traumatic - the episiotomy and huge blood loss, the anaemia which left me weak and drained, the slow recovery. I worry all the time that this one is big. I would definitely consider a caesarean if I knew for sure it was going to be as big as Ollie."
This is understandable, but caesareans leave you pretty drained, sore and immobile, too. I would choose the unassisted birth of my son, complete with tearing, bruises and incontinence over the caesarean I had with my daughter. But then again, at 8lb 7oz, he wasn't macrosomic.
According to Nahum you can influence your baby's size by trying not to gain too much weight during pregnancy, and taking plenty of iron (anaemia in pregnancy can lead to big babies, too). If you do suspect you are carrying a scrum half, though, here is a ray of light: a recent study found that bigger babies do better at school, have more stellar careers and are more likely to marry than small ones. So grit your teeth - it's worth it in the end.