The number of large babies weighing 10lb or more at birth has jumped by a fifth. What is behind this boom in hefty newborns? And how has it changed pregnancy and labour? Viv Groskop talks to mothers, doctors and midwives

Sarah Ockwell-Smith, 32, a hypnobirthing teacher from Saffron Walden, Essex, knows all about larger babies. At 5ft 1in, with a 6ft husband - Ian, 38, a locksmith - she has given birth to four. Sebastian, five, was 10lb (4.53kg) at birth. Flynn, four, was 9lb 12oz. Then they got even bigger. Rafferty, now three, was 11lb 3oz and Violet, one, was 11lb. "When I was pregnant with my first baby I remember a few of the babies from my antenatal class were around 9lb. I freaked out and said to my midwife, 'My baby is not going to be that big, is it?' She laughed and said no, he would be about 8lb."

But she actually thinks it is easier to give birth to a heavier baby. "My two biggest ones were by far the easiest: water births at home with no intervention. I would be more concerned about having a seven-pounder. With bigger babies, you have gravity on your side. And when they're born, it's nice to have big, chunky babies. You're not so worried about moving them and holding them."

The average birth weight of babies has been creeping up for the past 30 years. It is now around 7lb 8oz for boys, up 2oz from 1970, and 7lb 4oz for girls, an increase of 1.5oz, but the rise in babies with macrosomia (excessive birth weight) is more pronounced. According to figures from the Office of National Statistics, the number of babies more than 9lb 15oz increased by 20% in the decade up to 2003.

Definitions differ as to what constitutes a large baby. The previous threshold was 8lb 8oz. Now most health professionals class any baby more than 9lb 15oz as above average. Although larger babies do seem to be increasingly common, the heftiest are still rare enough to make the news. In May 2006 a 12lb 13oz baby was born in Aberdeenshire. Then, two months later the television presenter Kirstie Allsopp gave birth to her first child, Bay, at the St John & Elizabeth Hospital in north London. At 11lb 11oz, he was the heaviest baby ever born there.

Big babies used to be more common when people had larger families, simply because weight tends to increase with each subsequent baby. In January 2005 in Brazil, Francisca Ramos dos Santos gave birth to an 18lb "giant baby". He was her fifth child. Last year in Siberia, Tatyana Barabanov had a girl who weighed 17lb 5oz. She was a 12th child. According to the Guinness Book of Records the largest baby ever was born in Canada in 1879, weighing 23lb, but died 11 hours after birth. The heaviest surviving baby was born in Italy in 1955 and weighed 22lb 8oz. A 21lb baby was born in Cornwall in 1852 and a boy weighing 20lb 2oz was born in Crewe, Cheshire in 1894.

Society at large continues to find "sumo babies" hilarious. They are always an object of fascination. "I would get old ladies coming up to me in the street and saying to each other, 'Oh my God, have you seen this one, Maureen? He was 10lb 2oz,'" says Gina Robinson, 32, a retail manager from Sheffield. Her son Charlie is now seven months old. "The midwife actually took him out of the room when he was born and I heard her say to the other midwives, 'Look at the size of this one.'"

The mothers of larger babies are used to people wincing and saying "Ouch" when they tell them the baby's birth weight. "I really don't understand why society is so terrified of big babies. There is almost a freakshow aspect," says Ockwell-Smith. "But they do look slightly odd because they are so big. My friend had a 5lb baby at the same time as my 10lb baby and it was really funny seeing them together. Later, when the 5lb baby could sit up and move, mine was much bigger but could not move."

My own two babies were small fry compared with the Bay Allsopps of this world (8lb 4oz and 9lb) but all the same I felt curiously protective towards them because of other people's reactions. People laughed at my son when he was tiny because he was such a "bruiser" (code for "fat baby"). His granny nicknamed him "Beefy". An oversensitive new mother, I felt these comments were a tacit judgment on me: I had made these babies fat. Plus, I was slightly overweight during my pregnancies and I felt guilty about this. There is a sense that it is not acceptable to call the mother fat - but people betray what they really think of you by labelling the baby. As the midwife said to a colleague after weighing my 9lb baby: "By the size of the bump, I thought the baby was going to be massive. But it turned out to be mostly mum."

The reason babies are getting heavier is obvious, says Mervi Jokinen of the Royal College of Midwives: "It is mostly linked to better diet and living standards in the population. But big babies have always existed and sometimes it's very difficult to say if it's genetic or if it's diet." What is undeniable, however, is that obesity - of the mothers, not the babies - is a major factor. And with rising obesity rates comes an increase in gestational diabetes, which - unless carefully monitored - can increase the baby's weight. This is a form of diabetes that occurs purely in pregnancy, usually diagnosed after 28 weeks. After pregnancy, it disappears - although it is an indicator that the patient is more likely to develop diabetes later in life. Last month the Lancet reported that diabetes worldwide is "spiralling upwards" and now affects up to 5% of pregnancies.

The mothers of larger babies can at least draw some comfort from the idea that their baby's weight may be genetic or a fluke - not their "fault". Reassuringly, Dr Philip Owen, consultant obstetrician at North Glasgow University NHS Trust (biggest baby ever delivered: 12lb 1oz), says in some cases the mother's diet has no bearing on the size of the baby. "Obese mothers will have a higher incidence of larger babies. But babies weighing more than 9lb 15 oz are not the preserve of overweight mothers - they're also seen in 'normal' mothers too."

However, as veteran midwife Mary Cronk, who has birthed more than 1,600 babies during the past 40 years (including one 12lb baby), puts it in her no-nonsense way: "We are getting chubbier women these days." She would not want women to fear having a larger baby, though: "I think people get their knickers in a twist about big babies. If the labour progesses, then the baby will be born safely. After all, you can get problems with six-pounders. You need vigilance. There could be an advantage in having a home birth for a large baby. At least you have one-to-one care. In hospital you might be left on your own or get a third of a midwife." A bigger baby, she adds, is not necessarily a healthier one: "They can be pretty puffy, and they can lose weight quickly."

But the worry of having a large baby is becoming yet another stress factor in pregnancy. "I actually felt a bit guilty and embarrassed about having such a large baby," says Ruth Ainley, 34, a physiotherapist from St Albans. Her son James, now two, weighed 10lb 8oz at birth. When he was born by emergency C-section after a 28-hour labour, the surgeon announced, "Well, this one would not have come out any other way." "I thought it was a reflection on how much weight I'd gained and how much I'd been eating. I saw a consultant afterwards when I got pregnant the second time [Jonny, now five months, was 8lb 3oz] and he reassured me that it was perfectly normal.

"I did feel that everyone was going to think that I was a fat pig for producing such a fat baby. And you think, 'Are people saying that if you hadn't eaten so much in pregnancy, then he wouldn't be so big?' But I hadn't eaten whatever I wanted - I was careful. I had it in the back of my mind: I had better not eat too much, in the hope that it wasn't going to be a giant child." But in fact the size of her baby is hardly surprising: her husband is 6ft 4in.

Weight problems can affect women's choices in birth - to the extent that some severely overweight women find normal labour impossible. The rise in obesity, with its attendant complications, is at odds with the NHS's attempts to make "natural" birth and home birth more appealing. Maternity units are even having to request specialist equipment to deal with their larger patients. Last year in a University of Teesside study of 16 maternity units, hospital staff reported that they lacked larger delivery beds, sufficiently large blood pressure cuffs and longer needles for epidurals. Obese fathers caused problems too: one midwife said that it was embarrassing when they couldn't find large enough sterile "pyjamas" for men who wanted to accompany their wives into theatre.

Size remains a hugely contentious and emotional issue. Who wants to be told they're overweight when they're pregnant and can't diet anyway? In any case, many pregnant women who have gestational diabetes or large babies - or both - are not obese. And many obese women give birth to normal weight babies with no problems at all. The two concerns with larger babies are cephalopelvic disproportion (when the baby is too large to pass through the mother's pelvis) and shoulder dystocia (when the head comes out but the shoulders get stuck). But neither of these conditions is exclusive to larger babies: they can occur with small ones too.

If you are heavily pregnant with a very large baby, you can't win. On the one hand you are not allowed an elective Caesarean (because your birth could be "normal"). But on the other you may be advised against a home birth (because your birth could be "abnormal"). Despite the increase in numbers of these babies, it makes sense to adopt a "wait-and-see" policy in labour, says Dr Owen: "We can't accurately predict a baby's weight or the likely influence of the weight on the delivery. If you overreact you end up doing an awful lot of C-sections. And a planned C-section is safe but it is not risk-free." He says there is no denying that obesity is increasing in the general population - and in some cases this is affecting women's experience of pregnancy and labour as well as the weight of the baby. "Generally speaking, the bigger the baby, the longer the labour," he says, "and the greater the likelihood that you will need assistance with forceps or ventouse, or a C-section."

Although you can feel unusual for having a larger baby, you don't get many concessions in hospital, says Hana Downing, 37, a full-time mother of two from south-east London and pregnant with her third child who is due in July. Her second baby, Sam, now almost two, was 10lb 9oz: "When I was pregnant with him I looked ridiculous. Children were pointing at me in the street. When he came out he looked like a three-month-old baby with a wig on." Her first baby, Mya, now three, was also big for her gestational age: she was born at 34 weeks and was already 6lb 6oz. With her third baby, Downing has been diagnosed with gestational diabetes - which she suspects she must have had during her other two pregnancies too.

Despite this, she is expected - as are most women with larger babies - to go into labour naturally and "see what happens". This is usually hospital policy because it is so difficult to know the size of a baby before it comes out: palpation (the midwife feeling the bump) and ultrasound scans can be wrong by 10%. "I want an elective C-section because I don't want to be rushed into theatre," says Downing. "But they're saying that if we control the blood sugar [she is currently taking insulin], then I may have a perfectly normal-sized baby. I remain sceptical, having been through this twice."

Other mothers have felt the opposite pressure. Ockwell-Smith wanted to have her third child at home but was advised against it. "My consultant wrote on my notes, 'Home birth not advised because of baby's size.' I was told I was selfish and that I was putting both of our lives at risk. If I had not been so well-informed, I would have felt pressurised to do what they wanted." She had her baby safely and easily at home (all 11lb 3oz of him). "He took a long time to come out after his head was out. I think in hospital I would have had an epidural and been flat on my back: he would have got stuck and been diagnosed as a shoulder dystocia. At home I could move freely."

There are other practical problems, says Celia Dunsire, a midwife at West Middlesex Hospital in Isleworth, south-west London. Larger women do not have the freedom of movement that an efficient delivery demands, she explains. They cannot get into a position that is safe for them and for the baby: "A squat position is ideal. And because you've got abdomen meeting thigh, you can't squat deep enough." There is something very sad about this: it means some women are having to have assisted labours or having emergency C-sections simply because they are too large to bend over in labour.

It is easy to laugh at extremely large pregnant and labouring women - as well as at their gigantic babies. But in extreme cases the size factor can turn out to be a life-and-death issue. Last year the report Saving Mothers' Lives analysed the 295 deaths that occurred due to pregnancy or childbirth between 2003 and 2005. (To put this in perspective: the rate of deaths was 13.95 per 100,000 births.) Half the women who died were overweight or obese and 15% were extremely obese. Six of the women had a body mass index of more than 45, two of more than 60. ("Normal" BMI is 20 to 25.)

The NHS is currently facing a string of lawsuits from 246 families whose children are suffering from Erb's Palsy, an injury that can occur when shoulder dystocia is mishandled. Some - but not all - of these babies had excessive birth weight. If the current trend towards obesity in mothers and babies with heavier birth weights continues, then maternity units need to be able to cope with a potential rise in these types of problems.

Despite the possible complications, many mothers still argue that bigger babies are better. They report feeling more confident in looking after them, less concerned about their growth and relieved that they often sleep through the night at an earlier age. A University College London study conducted over several decades found that babies weighing more than 8lb performed better in IQ tests throughout their lives.

There are a lot of benefits, says Robinson: "Charlie has always slept well and had good control of his neck from the start. He never seemed like a tiny fragile baby and I wasn't as scared to pick him up as I might have been if he were tinier."

As Downing says: "Having had one child prematurely, it does make you feel confident when you have a baby who is big, bonny and healthy. You don't think, 'Oh my God, he's a giant.' He is still a newborn - just a big newborn".

· This article was amended on Tuesday June 10 2008. We were wrong to say in the sub-heading to this article that the number of babies weighing 10lb or more at birth had jumped by a fifth since 2003. As the article made clear, the figures related to the decade up to 2003. This has been corrected.