Quick answer: At 18 weeks your baby is about the size of a sweet potato, roughly 14.2cm crown to rump, with hearing well established and movements becoming more coordinated. Sleep often gets harder as your bump grows, so start practising side-sleeping now: the NHS advises sleeping on your side from around 28 weeks. A pregnancy pillow and a pillow between your knees help.
What parents need to know
Week 18 is often described as one of the most comfortable weeks of pregnancy. Your bump is showing, your energy is good, and you're likely feeling movements from your baby more frequently. But it's also a time when sleep can start to become a little more challenging as your bump grows — so let's talk about how to get the rest you need.
| Baby size | About a sweet potato |
|---|---|
| Crown to rump | Roughly 14.2cm |
| Side-sleeping from | Around 28 weeks |
| Next scan | 20-week anomaly scan |
| Baby's hearing | Ears fully formed, hearing established |
How big is your baby at 18 weeks?
Your baby is now about the size of a sweet potato — roughly 14.2cm from crown to rump. They're becoming more and more active each week.
What's happening with your baby?
Your baby's nervous system is maturing rapidly this week. The nerves are now being coated with myelin, a fatty protective substance that speeds up nerve signals. This means your baby's movements are becoming more purposeful and coordinated. Your baby is yawning, stretching, and may be sucking their thumb. The reproductive organs are now sufficiently developed that sex may be determinable at your 20-week anomaly scan.
Your baby's ears are now fully formed and hearing is well established. Research suggests babies can be startled by loud noises at this stage. Your baby's eyes remain sealed, but the retinas are becoming sensitive to light.
Sleep in pregnancy: what you need to know
Sleep challenges are common from the second trimester onwards. As your bump grows, finding a comfortable position becomes increasingly difficult. The NHS and sleep experts recommend sleeping on your side from around 28 weeks, as research suggests that sleeping on your back late in pregnancy may reduce blood flow to the baby and placenta. Starting to get into the habit of side-sleeping earlier in pregnancy makes this transition easier.
Left-side sleeping is often recommended as the optimal position as it improves blood flow to the heart and kidneys and helps nutrients reach the placenta, but sleeping on your right side is also fine. The key advice is simply to avoid lying flat on your back for prolonged periods from 28 weeks.
Tips for better sleep in pregnancy
A pregnancy pillow — a long, curved pillow that you can wrap around your body — can significantly improve comfort by supporting your bump, back, and hips simultaneously. Many pregnant women swear by them. Placing a pillow between your knees while lying on your side also helps reduce pressure on the hips and pelvis.
Other strategies that support good sleep include maintaining a regular sleep and wake time, keeping your bedroom cool and dark, avoiding screens for an hour before bed, and winding down with a relaxing routine such as a warm bath or pregnancy yoga stretches. Avoid large meals or excessive fluids close to bedtime, as heartburn and frequent urination are already common nighttime disruptions.
If you wake in the night and can't get back to sleep, try not to lie awake in bed for long periods. Getting up for a quiet, calming activity (reading, gentle stretching, warm herbal tea) and returning to bed when you feel sleepy again is often more effective than lying awake anxious about not sleeping.
Pelvic girdle pain and sleep
Pelvic girdle pain (PGP) can make sleeping particularly difficult, as turning over in bed, getting up, or being in any position for too long causes discomfort. A physiotherapy referral from your midwife can be enormously helpful. In the meantime, keeping your knees together when turning over, using a bed rail or rope to assist with getting up, and sleeping with a pillow between your knees can all reduce pain.
Your week 18 checklist
This week:
- Invest in a pregnancy pillow if sleep is becoming uncomfortable.
- Practise sleeping on your side.
- Manage any pelvic girdle pain with physiotherapy advice.
- Prepare questions for your 20-week anomaly scan.
- Continue to enjoy your baby's increasingly frequent movements.
Sources & further reading
- NHS — You and Your Baby at 18 Weeks Pregnant
- Tommy's — Sleep Problems in Pregnancy
- NCT — Sleeping Positions in Pregnancy
- Pelvic Partnership — Pelvic Girdle Pain Information
General information, not individual medical advice. Speak to your GP, midwife or health visitor about your baby’s needs.
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