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Can I predict my baby's birth weight?

It's inevitable that while you're pregnant you'll wonder what size baby you're expecting. There's no definite way to predict your baby's birth weight for certain, but there are plenty of factors to consider when making an educated guesstimate. Here's our stab at answering one of pregnancy's great unanswerables.

By Mumsnet HQ | Last updated Jul 2, 2021

Woman touch pregnancy bump

Baby growth spurts

Baby growth spurts are periods when your baby’s growth accelerates at an astonishing rate. It happens fairly frequently in the first couple of years of her life and sometimes it can feel as though she’s got bigger overnight. Often, she has done exactly that. It can be disarming (especially, when she’s outgrowing outfits every five minutes) and, while a growing baby is a healthy baby, you’d probably like to have some idea of what to expect.

Recently, a Mumsnetter asked: “Are growth spurts timed from a baby’s due date or her birth date?” It’s an interesting question, which caused plenty of discussion, and the answer is that growth spurts are timed from your baby’s due date, as they’re calculated based on her mental age from conception.

Can you predict your baby’s birth weight?

The question about growth spurts led to broader speculation about whether or not your baby’s birth weight can be predicted. It’s inevitable that, while pregnant, you’ll wonder just exactly who you’re carrying. Will they be tall, short, blonde, brunette, be able to sing, be good at sports? Inevitably, one of the most common questions is: What will my baby look like?

But what about birth weight – can it be predicted?

The short answer is yes and no. Not very helpful, we know. What we mean is that, while midwives and obstetricians can make estimates, based on ultrasound scans, these are not always accurate and can be wrong by as much as 10%.

Your midwife will measure your bump to get an idea of your baby’s growth. For first time mums, this happens at 25 weeks, while women who’ve given birth before without complications will be measured at 28 weeks.

I'm 34 weeks pregnant and my midwife felt the bump yesterday and said my baby will be about 8lbs at birth.

Your midwife will use a customised chart to follow your baby’s growth. She will remeasure your bump at subsequent appointments and, if it looks like your baby’s growth is slowing or she's growing more than expected, you'll be given an ultrasound, so that your midwife can get a more accurate idea of your baby’s size.

Sometimes, if a woman has had children already, her bump will be bigger because her stomach muscles have been stretched by a previous pregnancy. So, if you’ve had kids before, a larger bump doesn’t necessarily mean you’re having a big baby.

In short, there’s no exact way of predicting your baby’s weight. There are, however, key factors which could determine the size of the baby you're expecting:

Genetic factors in your baby’s birth weight

  • Parents’ weight. Your baby’s DNA carries the genetic information that will determine which characteristics she inherits from you and her dad. Of the 46 chromosomes your baby inherits, 23 come from you and 23 from him. In fact, your birth weight (time to dig out those yellowing certificates) is a factor in your baby’s birth weight (yours more so than the baby’s father’s).
  • Parents’ height. A tall baby – or a baby who grows up to be tall – is more likely to be on the heavier side. Height is one of the easier characteristics to predict, as it’s a polygenic trait (meaning no single gene acts alone). If you and your partner are both tall then your baby will be tall. If one of you is tall, and the other short, your baby will probably be of average height and weight.
  • Your age. There’s some evidence that teenage mums have smaller babies, while women aged 35-plus are more likely to give birth to bigger babies.
  • Premature birth. If your baby is born early then it follows that her birth weight will be lower than if she had been born at full term.
  • Multiple births. If you’re having more than one baby, it’s likely that they won’t be on the large side. There simply isn’t room in there for two big ones.
  • Mother’s medical issues. If you have anemia, diabetes or other medical conditions there is an increased risk that your baby will be born underweight. The same applies if you have any hereditary medical conditions. Discuss this with your doctor or midwife.

Environmental factors in your baby’s birth weight

  • Your diet. You need to eat healthily during pregnancy. Under eating will mean your baby doesn’t get the nutrients she needs and could be born underweight. On the other hand, there’s evidence from the Royal College of Obstetricians and Gynaecologists that the old cliche about expectant mothers “eating for two” is a load of twaddle. The really vital thing is that are careful about what you eat and make sure you consume an extra 200 calories per day in the final trimester.
  • Smoking and drinking alcohol in pregnancy can lead to complications for your baby and increase the chances of her being born underweight. The average birth weight is 7lb 8oz and the NHS says that babies born to women who smoke are, on average, 200g lighter than other babies. If you're expecting and trying to give up smoking then read the official advice on how and why to stop.
  • Altitude. Do you live up a mountain? If so, you should know there’s evidence to suggest that babies born at high altitudes tend to be smaller than average.

Do big babies need caesarean birth?

Wanting to predict your baby’s birth weight isn’t just about satisfying your curiosity. Having an idea of your baby's size could help you prepare for birth.

A baby who weighs more than 8lb 13oz at birth is considered oversize (macrsomia is the technical term for big baby). About eleven percent of babies in England are macrosomic and many women give birth to big babies without any problems at all. However, giving birth to a baby of 9lb15oz (very large) or more does increase the risk of complications.

My doctor said that, as my eldest son was 9lb 3oz, they would like me to have a growth scan at 28 weeks and 36 weeks to see if this one (also a boy) is on the large side too.

Giving birth to a very large baby can mean a longer labour. Twenty percent of very large babies require an assisted birth, involving instruments, such as forceps, that are attached to the baby’s head. There’s also the risk that you’ll suffer a perineal tear and heavy blood loss.

There’s a one in 13 chance that a very large baby will suffer a shoulder dystocia, by getting stuck during pushing. This can cause nerve damage from which she should make a full recovery.

For these reasons, some doctors recommend caesarean birth for babies who shows signs of being very large. If you’re worried about giving birth to a very large baby then it might help to write about it in your birth plan. In the meantime, try to remember that big babies are born vaginally every year with no complications at all and go on to be healthy (not so little) people.