Health: The big baby boom

Barbara Bertagnolli was stunned when her second baby, Lucian, was born. "He was massive. He weighed 4.565kg [more than 10lb] and measured 55cm [nearly 22in]. I had no idea he was going to be so big. After a scan at 36 weeks they said he was a normal size."

Mrs Bertagnolli, a 37-year-old goldsmith, recalls: "The day before I went into labour my tummy was so hard I thought I was starting contractions, and this old, wise-looking midwife came along, felt my tummy and said the hardness was `all baby'. She said it was a big, big baby.

"There was a problem coming out - his head was so large, it stuck for 10 minutes. It wouldn't come out with one contraction. It was half in, half out."

Lucian is now 11 months, and still very large. "I had him weighed at nine-and-a-half months and he weighed 13.4kg. In the last two months he's actually gone off the growing chart. He's not really fat, just big, although he has got chubby cheeks and thighs. He's still got his baby face - but he looks like a two-year-old.

"The health visitor thought he might need to go on a diet and referred me to the doctor. The doctor said it was the third really big baby he'd seen that morning - which came as something of a relief to me."

Being told you have given birth to a "whopper" is guaranteed to inspire a glow of pride in the most exhausted new mothers - and fathers, too. Big babies are traditionally a cause for celebration. They are thought to be healthier, stronger and less vulnerable to infection, while recent research suggests that as adults they may lead longer, more disease-free lives.

However, some specialists believe that British babies may be growing too large. They say a large baby poses a higher risk of complications in labour, perineal damage to the mother and emergency Caesarean surgery.

"The number of Caesareans is rising," says Rupert Fawdry, a consultant obstetrician at Milton Keynes hospital. "The usual reason is failure to progress in labour. And the most likely reason for this is that the baby is too big for the mother's pelvis."

There is little doubt that British babies are getting heavier. According to the Office for National Statistics, the proportion of newborns weighing 3,500g (about 7lb 11oz) and over has been steadily increasing over the last 20 years. The number of very heavy babies has also risen: the proportion weighing between 4,000g and 4,499g (8lb 13oz-9lb 15oz) jumped nearly 2 per cent between 1982 and 1996, while those weighing 4,500g (about 10lb) and over rose from 1.09 per cent to 1.63 per cent in the same period.

An increase in babies' birthweight is seen by many researchers as a positive sign, reflecting improvements in the health of the adult population.

"Bigger babies are getting a better mix of nutrients and oxygen while in the womb," says Professor David Barker, an epidemiologist with the Medical Research Council at Southampton General Hospital. "This is because women are getting healthier and bigger, and are therefore able to pass on the right nutrients to the foetus." A baby's size at birth, he points out, is determined not by the parents' genes (which kick in after birth) but by conditions in the womb.

Larger babies have always been associated with lower infant mortality rates, while recent research by Professor Barker indicates that they run a lower risk of heart disease, stroke and hypertension in later life.

But is bigger always better? Rupert Fawdry is unconvinced. He says that big babies pose a higher risk of complications in labour and of damage to a mother's back passage, which can result in faecal incontinence. He also points out that the number of women having Caesareans has risen dramatically; the figure is almost three times higher than it was 20 years ago.

"What is making it so difficult nowadays for women to deliver vaginally?" he asks. "Of course there may be many reasons for the rise in Caesareans - including fear of litigation - but one factor could be big babies." In addition, he says that a woman who has already had a difficult labour with one large baby may well be inclined to opt for an elective Caesarean next time.

But why should large babies cause problems in labour, if mothers are getting bigger too? Dr Fawdry suggests that the pelvic bones may not be affected by an increase in height. "Women may be getting bigger, but they are still not big enough compared to the size of the babies," he argues.

Large babies may also be affected by a difficult birth. Dr Vanda Joss, a paediatrician in the special baby care unit at Milton Keynes, says they are more likely to get stuck in the birth canal and suffer dangerous oxygen shortage. Another common problem is dystocia, where the head is out, but the shoulders get stuck. This can, says Dr Joss, also cause oxygen deficiency (because the cord gets squashed) and commonly results in permanent damage to nerves in the arm.

But don't big babies turn into healthy adults? Although research shows heavy babies to have lower rates of heart disease as adults, Professor Barker says there may be a trade-off. "There is evidence to suggest a link between large babies and higher rates of breast, prostate and ovarian cancer in adults," he says.

One problem with any discussion of big babies is that nothing is known about the shape of this new generation of heavyweights, since records of height and head circumference are not kept nationally. And shape is all-important. "A long, muscular baby is good," says Professor Barker, while a short, fat, heavy baby is unhealthy, and normally the product of a fat mother.

But Dr Fawdry questions this distinction, pointing out that most of the big babies he sees are fat, too. He suggests there may be a link between the "explosion" of diabetes (including in pregnant women) in the UK and other Western nations, and the increasing numbers of big babies.

"There is an assumption that big babies are the result of improved health and nutrition, but we don't really know this," he says. "Maybe we are worse off nutritionally. Perhaps something in our diet is causing both diabetes and big babies.

"We don't know yet what the correct diet ought to be to produce the right size of baby - one that doesn't need a Caesarean, or cause damage to the mother."

So is there an optimal weight for a baby? This partly depends on the mother's size, but Dr Fawdry says there is evidence to show that babies of 3,400g (7lb 8oz) have a prenatal mortality rate half the national average. "Perhaps this is the ideal weight - not too big and not too small," he suggests.

"The most important breakthrough in maternity care in the next decade will be to understand how a baby's size can be controlled - so that it won't end up too big or too small. It may be diet, vitamins, or hormones that hold the key, but if we can find how to do it, it could utterly transform women's experience of childbirth."

Professor Barker, however, still feels that, overall, bigger has to be better. He points to Sweden, a country with one of the highest median birth weights in the world. "Many of the women are over 6ft and very muscular and babies of 5,000g (11lb) are not uncommon," he says. He believes that this is not because the Swedes are larger as a race, but because of improvements in health and nutrition. "A big baby is a sign of optimal development and a healthier future."