Giving birth to a big baby can be a common worry for pregnant women. I have lost count of the hundreds of women who have come through my classes over the years and been diagnosed with a suspected big baby. If the ‘big baby’ box has been ticked you may consider scheduling a caesarean section or early induction. However, most big babies are born healthy and your birth plan may not be affected in any way.
What is considered a big baby?
A big baby is a baby born over 4Kg. A big baby is a baby expected to be born on the 90th percentile. This means a baby that weighs more than 90% of babies born at the same age in the population.
How accurate are these estimates?
Nobody will know for sure what your baby will weigh until they are put on the weighing scales after birth.
Ultrasound is believed to be the most accurate method of estimating the weight even though there is a significant margin for incorrectly predicting the weight. In fact, ultrasound could in fact do more harm than good by increasing the rates of intervention. Ultrasound scans are good for plotting and monitoring growth over a period of time, but not for an accurate one-off measuring tool. If you’re offered a growth scan you don’t have to have one, you can politely decline!
I know many stories where mums were told to expect an average-sized baby and went on to have a far larger baby. Likewise, mums who were told that they were expecting a big baby who actually had a baby of average weight!
Does a big baby mean a difficult birth?
No, not necessarily. Having a larger baby does not necessarily mean labour will be more complicated despite the perception that it will be harder. Some women have no problem birthing a 10lb baby, whereas others may struggle with a 6lb baby. The majority of women who have a large baby will have a normal vaginal birth with no complications.
Neither your pelvis nor your baby’s head is a fixed object! The female pelvis has evolved and is designed specifically for birth. It is designed to open up and make space for the baby. Your pelvis is made up of several bones which are held together by ligaments. During pregnancy, your body produces a hormone called Relaxin which softens and loosens those ligaments to facilitate the baby’s movement through the birth canal.
Similarly, your baby’s skull is made up of several bones that allow their head to change shape and mould so that it fits through the pelvis and birth canal with ease. This is why your baby has soft spots called fontanels. Babies know how to get into a good position for their birth, they tuck their head in so that the smallest part of their head presents first.
If you’re on the petite side and/or expecting a big baby you may hear that the baby is too big to fit through the pelvis. When a baby cannot fit through the pelvis it’s called Cephalopelvic Disproportion (CPD). True cases of CPD are thankfully rare and it is often a result of injuries or an abnormality. I’ve only encountered this once in many years of teaching and it involved a lady who had fractured her pelvis as a child. CPD is nearly impossible to diagnose before labour is well established unless you have a known injury or abnormality.
Does an early induction prevent issues if it is a big baby?
Research has shown that there was very little difference in the weight of the babies born after induction. The average difference in weight was roughly 178g! Guidelines state that induction of labour should not be carried out simply because a baby is suspected of being big.
Research shows that any complications may be due to the interventions carried out when a baby is suspected to be big, rather than because baby is big. Inducing a mother before her baby and her body is ready for birth increases the risk of birth becoming problematic. It is the management of labour; being kept immobile whilst being induced, on a drip, or constantly monitored, that is the problem.
The size of a baby in normal physiological birth circumstances is irrelevant as long as the mother is able to move around freely. Being immobile on a bed significantly reduces her pelvic capacity by an average of about 30%. Care providers are more likely to diagnose slow progress during labour and recommend a caesarean if they suspect the baby is big. Women with suspected large babies are twice as likely to have medical interventions.
Why are some babies bigger?
Below are some of the main reasons that a baby could be slightly bigger than average.
- It is not your first baby; subsequent babies tend to be a little larger.
- You previously had a large baby.
- The length of your pregnancy; if pregnancy is longer the baby may be bigger as they keep growing.
- If the baby is a boy.
- Because of the size of the parents.
- Because of any weight you gain during pregnancy.
- Gestational diabetes.
It is important to add that overweight women are just as likely to give birth to small babies. Even if some of the above risk factors are present, there is no guarantee that it will result in a big baby.
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