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Large for gestational age: what expectant mothers should know

Posted: 01/06/2023

There is no set definition for babies that are ‘large for gestational age’ (LGA), but the term is normally used to refer to babies that are over the 90th percentile. This means that the baby weighs more than 90% of babies at the same gestational age in the population.

Approximately 5% to 8% of babies born in the UK are identified as LGA.

How do you detect LGA babies?

During pregnancy, a woman will have a customised growth chart prepared for her, based on information such as her height and weight, the number of babies she has had (and their weights) and her ethnic group. On the basis of this information, a growth chart will be drawn up. The lines on the growth chart are called centiles (or percentiles) and will show the expected pattern of growth for her baby during the ongoing pregnancy.

At every antenatal appointment after 26 weeks, the distance between the top of her womb and the bone at the front of her pelvis is measured (the symphysis fundal height). This measurement is then plotted on her customised growth chart. If this measurement is greater than expected on two or more occasions, she will be offered an appointment at a clinic for an ultrasound scan to assess her baby’s weight. From this scan, it will usually be possible to determine whether a baby is LGA.

It is important to note that an error rate of 15% is commonly accepted when estimating fetal weight based on an ultrasound scan.

What causes a baby to be LGA?

It is often not clear why a baby is LGA.

However, statistics show that the following women are at increased risk of having a large baby:

  • Women who have had a large baby before, or have a family history of large babies.
  • Women with a body mass index (BMI) of 35 or above.
  • Women who are known to be diabetic or who develop diabetes during pregnancy – high levels of sugar in the blood can pass through the placenta to the baby and affect growth.

What are the risks of having a LGA baby?

It is important to remember that, although the risks are increased, most babies identified as LGA are born vaginally with no complications.

However, there are some rare but significant risks to mother and baby where the baby is identified as LGA, especially when the birthweight rises above 4.5kg.

  • The baby’s shoulders may struggle to come through the pelvis (this is called shoulder dystocia), which occurs in 68 out of every 1000 births in women with suspected LGA babies. Very rarely, shoulder dystocia may lead to birth injuries, including fractured clavicle (collar bone), Erb’s palsy (upper arm nerve damage), brain damage or, incredibly rarely, death.
  • An increased risk that the mother will have a large blood loss after the birth (postpartum haemorrhage).
  • An increased risk of the mother having a bad tear of the perineum, requiring stitches in theatre under anaesthetic.

In February 2021, the Healthcare Safety Investigation Branch (which looks into the safety of NHS practices in the UK) published a national learning report on the themes emerging from its maternity investigations. It concluded that many of the challenges detailed above arise due to issues in how LGA babies are identified and, if they are identified, the management of care provided to the mother during pregnancy and delivery.

Is there any recommended treatment to avoid complications with a LGA baby?

Unfortunately, research into the best management plan for LGA babies is not well established. As a result, there is an array of varying national guidelines regarding diagnosis and management. This means that doctors are less able to provide recommendations for care and treatment.

To help improve clinical data on the best management for LGA babies, mothers expecting LGA babies should consider agreeing to be part of ‘The Big Baby Trial’, which is investigating the question of whether induction of labour is of benefit, and at what correct gestational age it should be offered.

However, while research on the best management for LGA babies is not yet well established, this should not prevent doctors from sharing information with expectant mothers to support them in making an informed choice about their birth preference. Most guidelines are clear that the options which should be discussed with mothers of LGA babies are:

  • early induction of labour; and
  • caesarean section (particularly for babies above 5kg).

Once a doctor has informed the mother of an LGA baby of her options, she should be given the opportunity to discuss her wishes in respect of delivery and, so far as is possible, these should be followed.

Rosa Shand, associate in the clinical negligence team at Penningtons Manches Cooper, comments: “Sadly, the HSIB report makes painfully clear that there is a pressing need for research into big babies, so that mothers can be better informed about risks and the best management to ensure safe delivery. Currently, it seems that the lack of evidence in this area is paralysing doctors, who feel unable to confidently counsel expectant mothers. This means that mothers are sometimes left in the dark. At Penningtons Manches Cooper, we see many sad cases where mothers have not been consulted or their wishes not taken into account, resulting in devastating consequences, such as a birth injury or stillbirth.”

If you or a family member have any concerns regarding care provided during childbirth, the Penningtons Manches Cooper team are here to provide specialist advice. Please contact us on freephone 0800 328 9545, email or complete our online assessment form. An initial, free, no obligation meeting will then be arranged.

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Penningtons Manches Cooper LLP is a limited liability partnership registered in England and Wales with registered number OC311575 and is authorised and regulated by the Solicitors Regulation Authority under number 419867.

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