What size is considered to be large for gestational age?

Large for gestational age (LGA) is a term that describes a baby who is larger than most other babies of the same gestational age. The gestational age of a baby needs to be thought about when deciding on the expected weight for a baby. The “normal” range has been set by looking at very large numbers of babies born and recording their weights and their gestation at delivery. The average measurement will fall on the 50th percentile (percentile means that the measurements are plotted on a 100 point-scale).

A measurement that falls on or above the 90th or sometimes 95th percentile — meaning that the baby’s size is greater than 90% or 95% of babies of that gestational age — suggests a baby is large for gestational age (or macrosomic). Sometimes a baby’s growth may suddenly accelerate, e.g. if the baby was growing along the 40^th centile but suddenly jumps up to the 70-80^th centile. Such a baby may also be considered as gaining too much weight even though it technically measures below the 90^th centile.

How is a large baby detected?

An LGA baby is detected in the same way as a baby who is small for gestational age.
During most antenatal appointments, your midwife or doctor will check the size of your uterus by feeling your abdomen (this is called an abdominal palpation) and measuring where the top of the uterus is in relation to your pelvis and rib cage “the symphysio-fundal height or SFH” – this gives an indication of the size of the baby. Usually this measurement is plotted on a chart in your antenatal notes. If there is a suspicion that a baby may be growing too quickly or too slowly, then your doctor or midwife will likely perform an ultrasound to check the size of the baby.  Measurements of a baby’s thigh bone length and head and abdominal circumference are made, and these measurements are plotted on a graph and compared to the size of the “average” baby of the same gestational age.

What is macrosomia?

Macrosomia is a term used to describe a baby who is significantly larger than average. Whereas large for gestational age relates to a baby’s size before birth, macrosomia is usually used to describe babies following birth who are larger than the 90th or 95th percentile on an infant growth chart or who weigh 4kg or more at birth.

What causes an LGA baby?

Many factors influence a baby’s birth weight and in utero growth rate. Genetic material from the baby’s biological mother and father influence size, so larger mothers and fathers tend to have larger babies, but this genetic influence seems to be limited; across the world most well-nourished mothers who have uncomplicated pregnancies will give birth to similar sized babies. There are some factors, however, that increase the risk of a baby being born large. These factors include: the mother being overweight or obese or gaining a large amount of weight in pregnancy; having gestational diabetes; having had a previous large baby; or having a pregnancy that lasts longer than 40 weeks. If a large baby is detected on a scan, the mother may be tested to see if she has developed gestational diabetes. A woman who has diabetes or who develops gestational diabetes is at most risk of having a large baby, especially if her glucose levels are difficult to control. For this reason, women who have diabetes will be offered scans more often than women who do not.

Is it possible to prevent macrosomia?

The risk of having a baby who is large for gestational age can be reduced by being in the best possible shape before birth. Briefly, that would mean maintaining a healthy weight, eating a healthy diet, not gaining an excessive amount of weight in pregnancy and being moderately active. Women who have diabetes or who develop gestational diabetes should aim to keep their glucose levels consistently within normal levels.

Why is having a large baby a concern?

Most babies, even large babies, will have no problems; however, larger babies are at greater risk of suffering difficult births because their size can interfere with the normal birth process. Labour is less likely to start spontaneously which means labour induction is required more often. At birth weights above 4kg, Caesarean or instrumental birth are much more likely and sometimes there may be difficulties during birth; very rarely the baby’s shoulders may struggle to come through the woman’s pelvis (this is called shoulder dystocia). Babies above 4.5kg are at much higher risk. Doctors and midwives are specially trained to deal with this situation and, with some simple actions, most babies deliver normally. However, very rarely, shoulder dystocia may lead to birth injuries including a fractured clavicle (collar bone) or Erb’s palsy (upper arm nerve damage).

Large babies also put mothers at higher risk of significant bleeding after birth (post-partum haemorrhage), infection in labour (chorioamnionitis) and tears involving the bottom (3^rd/4^th degree tears).

Deciding on how to deliver an LGA baby

 

At present there is no evidence to suggest that non-diabetic pregnant women with an LGA baby need to have an elective caesarean section — the exception is if the estimated weight of the baby is more than 5kg.

Some doctors may recommend an induction of labour when the baby is at term. This is because the baby is ready to be born — if the baby is born after the due date it will only continue to gain extra weight. This is a good option for women who have had a baby before as it is not linked with an increased risk of caesarean section in labour. When deciding on how and at what gestation to deliver a baby, doctors will take into account the mother’s weight, age, height, previous babies and previous deliveries before making any recommendations. Women with diabetes will have different recommendations which are mainly related to the diabetes itself.

Are there any immediate care requirements for LGA babies after birth?

All babies are assessed at birth and cared for according to that assessment. Most large babies will need the same care as babies of an average weight and size. If there have been difficulties at birth, including shoulder dystocia, or the mother has diabetes, increased observation of the baby may be needed to make sure they are coping well with life outside the uterus (the womb). These observations may be made on the maternity ward, or the baby may be admitted to the neonatal unit where it can be observed very closely. A large baby may require feeding more regularly, particularly if its mother has diabetes.

Are there any longer-term complications for large babies?

Most babies who are large at birth will have no longer-term complications and should be treated like any other baby. There is some evidence, however, that babies born large at birth are more likely to become overweight or obese and may be more likely to develop diabetes and cardiovascular disease in later life. It is possible to reduce these risks by breastfeeding. Again, like average weight infants, large for gestational age infants once weaned should receive a healthy diet and be physically active. The adoption of a healthy lifestyle in early infancy and its maintenance throughout life is important.

Main Points

• Large for gestational age (LGA) is a term that describes a baby in the womb who is larger than 90-95% of other babies of the same gestational age.
• A large baby is usually detected by your midwife or doctor during your antenatal appointments. They can measure where the top of your uterus is, and therefore have a good indication of the size of your baby. An ultrasound scan can confirm how the baby is developing.
• Macrosomia is a term used to describe a baby after birth who is significantly larger than average.
• Many factors influence a baby’s growth in the uterus and may contribute to a high birth weight. These include genetics (though their influence is limited), a prolonged pregnancy that lasts over 40 weeks, pre-pregnancy weight or a significant amount of weight gain by the mother during her pregnancy, and gestational diabetes.
• The risk of having a baby who is large for gestational age can be reduced by having a BMI in the ideal range, eating a healthy diet and being moderately active.
• Most babies, even large babies, will have no problems; however, larger babies are at higher risk of suffering difficult births because their size can interfere with the normal birth process. For instance, shoulder dystocia, where the baby’s shoulders get stuck behind the mother’s pelvic bones, is more common with a larger than average baby.
• A large baby may require feeding more regularly, particularly if mum has diabetes.
• There is some evidence that babies born large at birth are more likely to become overweight or obese and may be more likely to develop diabetes and cardiovascular disease in later life.