Macrosomia or Having a Big Baby

Macrosomia is the clinical term meaning 'big baby'. The average weight of a newborn baby is 7lbs 8ozs, but babies who weigh 9lbs 15ozs or more at birth are said to be macrosomic.

How is macrosomia diagnosed?

The science of predicting a baby's weight before it is born is notoriously difficult. During pregnancy, a midwife will regularly measure your bump using a tape measure. They measure from the top of your uterus, just under your ribcage, down to the top of your pubic bone. A reading is taken in centimetres with the idea being that the measurement should corresponds with how many weeks pregnant you are. So, for example, if you are 32 weeks pregnant, then your bump will be expected to measure 32 centimetres. Sometimes, if your bump is measuring significantly larger than expected, your midwife may be inclined to suspect that your baby is big. You may be referred for an ultrasound scan to take further measurements of the baby. At the hospital, a sonographer will measure the circumference of the abdomen to determine the size of the baby and for accuracy. This will be done after the 30th week of pregnancy. In a small number of cases, a baby will be found to be abnormally large for its gestation, but more often than not, babies are of an average size and no further investigations are necessary.

It is very difficult to accurately determine the true size of a baby while it is the womb. Some women carry a lot of amniotic fluid (the watery substance surrounding the baby) and this causes a pregnancy bump to measure on the large side. It also depends on the position you are in while you are being measured by a midwife. Other factors such as bloating can also skew the measurement.

What causes macrosomia?

Many factors can lead to a true case of macrosomia. Generally, if there is a family history of large babies, then the cause is merely genetic and innocuous. Boy babies also tend to grow larger than girls in the womb and statistically, you are more likely to give birth to a large baby if your previous babies were bigger than average. Medical complications in pregnancy such as gestational diabetes can lead to macrosomic babies. This condition requires prompt treatment and medication to control blood sugar levels in both mother and baby. If it is determined that your baby is big and this growth has been caused by gestational diabetes, you may be induced to allow you to give birth before the baby grows further and thus trying to avoid the need for a caesarean section. Other factors associated with macrosomia are obesity, excessive weight gain during pregnancy and if you are more than two weeks overdue. These circumstances alone do not predetermine a big baby and it is sometimes the case that women with no risk factors give birth to larger than average babies.

What are the risks associated with having a big baby?

Giving birth to a larger baby need not necessarily cause any problems during labour and birth. A baby's head is soft and pliable and it is designed to be squeezed down the birth canal to allow it to be born. Even a big baby will do just this. However, there is an increased risk of developing difficulties during birth which may result in tearing and medical intervention such as an episiotomy or assisted deliveries using forceps or ventouse. Despite this, a vaginal birth is definitely possible. Some overly cautious doctors and midwives may recommend a caesarean section if there is concern that the baby will be very big, but ultimately it is your choice whether to go for a section or try for a normal delivery.

In a small percentage of births involving macrosomic babies, shoulder dystocia may occur. Although rare, this constitutes a medical emergency and needs to be dealt with immediately as it is potentially a very serious situation. Shoulder dystocia is where a baby's shoulders get stuck behind the pubic bone after its head has been pushed out. In dealing with this, doctors and midwives will manipulate the baby and you may need to change positions or have an episiotomy to get baby out safely. It is not uncommon for babies with shoulder dystocia to suffer a few problems afterwards. Sometimes, damage can be caused to the nerves in the shoulder and arm or their collarbone maybe broken. However, in either scenario, these injuries will fully heal and there are unlikely to be other long term consequences that a big baby will suffer.

Your health following the delivery of a big baby may be affected. You may well be recovering from an episiotomy or tearing and it is important that you look after yourself. Keep the area clean to allow wounds to heal well and avoid them getting infected. If gestational diabetes caused macrosomia, blood sugar levels should start to regulate after giving birth.