What’s considered to be a ‘big baby’?
In the UK a ‘big baby’ (medically known as macrosomia) is considered to be a baby that is 4kg (8.8lb) or over at birth. Or a baby that’s predicted to be over the 90th centile on the growth chart (i.e. weighs more than 90% of the babies who are of the same gestational age).
During pregnancy, at your regular midwife checkups, your midwife will measure your bump with a tape measure from the top of your uterus to your pubic bone. This is called the symphysis fundal height (SFH). This can give an idea of the size of your baby, but it’s more of a gauge than an accurate measurement.
If this measurement shows the baby’s growth to be above the predicted trajectory on your growth chart, you will be offered a growth scan. The growth scan may then pick up that the baby is LFGA (large for gestational age). However, we also know that these scans have a 15% margin for error. So if you have a baby that is predicted to be 4kg (8.8lb) it could be anywhere from 3.4kg (7lb 5oz) to 4.6kg (10lbs 4oz) - which is a huge range! It’s worth taking your baby’s predicted weight from your scans and working out what your range is by calculating the +/- 15% rule.
Gestational diabetes
One common cause of a bigger baby is Gestational Diabetes (GD), a condition where the mother has high blood sugar levels during pregnancy. Women who have uncontrolled blood sugars may have a baby that carries more fat around the shoulders, making it a bigger baby, so if your baby is predicted to be LFGA (large for gestational age) your midwife will offer you a Glucose Tolerance Test to check for diabetes. If you do have diabetes in pregnancy you will be put under the care of the diabetic team and timing of delivery will be discussed with you along with more regular scans.
Why else might you have a big baby?
Well, we expect healthy people, who don’t have any medical problems, to grow nice-sized, healthy babies! Bigger women often birth bigger babies and bigger babies can follow on through families. Second babies are known to be slightly bigger than the first too. Some statistics suggest that in the UK, over 10% of babies are born over 4kg and each day, thousands of women give birth vaginally to ‘big babies’.
What are the risks associated with giving birth vaginally to a big baby?
If you have had an uncomplicated pregnancy with no complications during a previous birth, you remain at a lower risk of complications. One of the main risks that will be discussed with you about giving birth vaginally to a big baby is shoulder dystocia. This is where the shoulders get stuck on the mother’s pubic bone after the head is born. Midwives and obstetricians are well trained in dealing with this emergency, using special manoeuvres of the mother and baby to help deliver the baby safely and quickly, however, there are some risks. Shoulder dystocia can lead to perineal damage and bleeding and there’s a risk of injury to the nerves around the baby’s shoulder and arm. However, the baby is likely to make a full recovery. If the shoulder dystocia is severe, a small amount of babies can have some brain damage, but this is very rare.
What are your options if you’re told you have a big baby?
If your baby is predicted to be LFGA, depending on where you are in your gestation, your baby’s growth and your health, you may be offered an induction at some point during your pregnancy. This might be an intervention that wasn’t originally on your birth preferences and so you might be wondering how to decide on the best course of action for you and your baby.
This blog does not replace the medical information and advice you receive from your team, but you can be prepared with the knowledge to help empower you to ask questions and make decisions in different situations.
For women that do not have Gestational Diabetes, there has been no evidence to suggest that inducing labour due to a big baby has a proven benefit for the mum or baby. This doesn’t mean you should or shouldn’t accept the induction, as there may be reasons within your own situation that make it the best decision for you. It’s just that we don’t have the data to say one way or the other! A big baby trial is currently being carried out to help clarify this matter.
Generally, discovering you have a big baby does not result in any emergency, so the good news is, there’s no rush to make a decision! There’s plenty of time to speak to your care providers, do your research and talk to your birth partner or other members of your support network. Gather all the information you can, so that whatever decision you make is the right one for you and your family.