Having a Big Baby (Large for Gestational Age)

Having a Big Baby (Large for Gestational Age)

Patient information

Obstetrics

• Author ID:           ZH 
• Leaflet Ref:         Obs 094 
• Version:              1
• Leaflet title:         Having a Big Baby (Large for Gestational Age)
• Date Produced:   December 2025 
• Expiry Date:        December 2027 

What does “Big Baby” mean?

Some babies grow larger than expected in pregnancy. This is sometimes called:

Large for Gestational Age (LGA) – when your baby’s size is above the 90th centile on a growth chart at or after 36 weeks of pregnancy. 

Macrosomia – when a baby is estimated to weigh more than 4.0–4.5kg (8lb 13oz–9lb 15oz) at birth.

Why are some babies bigger?

There are many reasons why babies grow larger, including:

• Family traits (taller or larger parents)
• Mother’s weight or weight gain in pregnancy
• Diabetes in pregnancy (gestational or pre-existing)
• Being overdue (born after 40 weeks)
• Lifestyle and health factors

How do we check for a big baby?

• Your midwife measures your bump (fundal height) at each visit.
• If the measurements grow faster than expected, you may be offered a growth scan.
• A scan can estimate your baby’s weight, but this is only an estimate.

If one fundal height measurement (SFH) identifies your baby’s weight as estimated >90 centile, then this would not be an indication for an ultrasound scan 

If the SFH continues to suggest accelerated growth of your baby, you will be sent for an ultrasound scan to estimate your baby’s weight; this is to rule out increased water around your baby or a big baby

If the ultrasound scan shows your baby’s weight is estimated >90th Centile and/or >4kg, then a ‘large for gestational age’ (LGA) baby or macrosomic baby is diagnosed; in this case, we recommend a screening test for Gestational diabetes within 4 weeks; Gestational diabetes can be a common and modifiable cause of LGA/macrosomia, and/or increased amniotic fluid 

If Gestational diabetes is confirmed, you will be referred to our Specialist Diabetes Midwife and Diabetic Antenatal clinic.

It is important to know that scans and clinical estimates of a baby’s size are not always exact. They can differ by 10–15%. This means a baby thought to be large may be an average weight at birth.

What are the possible risks of a big baby?

Most big babies are born without problems, but there are some increased risks:

For the mother:

• Longer or more difficult labour
• Need for assistance (ventouse, forceps, or caesarean)
• Heavy bleeding after birth (postpartum haemorrhage)
• Deep perineal tears

For the baby:

• Shoulder dystocia (baby’s shoulder getting stuck after the head is born – an emergency, but the team is trained to deal with this)
• Rare risks of broken bones or nerve injury
• Low blood sugar after birth (especially if a mother has diabetes)
• Intrapartum stillbirth (very rare)

What choices do I have?

If your baby is suspected to be large, your options will be discussed with you. These may include:

1. Expectant management (waiting for labour to start naturally)

• Low-intervention approach.
• Safe if no other complications.
• You will continue your usual antenatal care.

2. Induction of labour (starting labour with medication)

• May be offered from 38 weeks.
• Evidence shows that it may lower the chance of shoulder dystocia and reduce some complications.
• Does not appear to increase caesarean risk.

3. Planned caesarean birth

• Usually discussed if your baby is estimated to weigh 5kg (11lb) or more, or if you prefer this option.
• Normally recommended at 39 weeks.

Your healthcare team will support you to make the decision that feels right for you.

Place of birth

• If your baby is above the 90th centile on scan, it is recommended that you give birth in a consultant-led maternity unit.
• This ensures rapid access to a skilled team and emergency care if needed.
• If you prefer another setting (midwife-led unit or home), your midwife will discuss the risks and support your informed choice.

During labour

• Monitoring: Intermittent listening to your baby’s heartbeat is recommended, unless there are other concerns.
• Positions: You can usually move around, and waterbirth is not contraindicated.
• If labour slows: Your care will be reviewed by a senior doctor before extra medicines (like oxytocin) are used.
• Second stage (pushing): If the baby is not moving down, the team may suggest an assisted delivery or caesarean section.

After birth

• You will be offered active management of the third stage (an injection to help deliver your placenta) to reduce the risk of heavy bleeding.
• Your baby may need extra checks for blood sugar levels, especially if you had diabetes.

Key points to remember

• Most women with a suspected big baby have safe and straightforward births.
• Scans are not 100% accurate – some babies predicted to be big are average-sized at birth.
• If induction or caesarean is offered, you will have time to discuss the benefits and risks.
• Your preferences and choices matter – your team will support you.

Where to find more information

• NHS Choices: www.nhs.uk/pregnancy
• Perinatal Institute: www.perinatal.org.uk
• Big Baby Trial 2025: https://pubmed.ncbi.nlm.nih.gov/40319899/
• Speak to your midwife or doctor if you have any concerns.

This leaflet is for general information only. It does not replace personalised medical advice. Please speak with your midwife or doctor about your own situation