21 weeks pregnant: the second half begins
The start of a fast-growing stretch. Here is how to stay steady, rest well and find support if your scan brought news you did not expect.
Updated October 13, 2026 · sources checked October 13, 2026
In short
At 21 weeks your baby is about 26.7cm from head to toe, now heavier than the placenta, and can hear voices outside the womb. Your changing centre of gravity and looser joints can make you unsteady. This week, book antenatal classes if you would like them, and tell your midwife or doctor about any fall, even a small one.
Where you are at 21 weeks
You have started the second half of pregnancy, and Cleveland Clinic's month six (weeks 21 to 24). The NHS calls the coming weeks a period of rapid growth. Check your exact week and day with the pregnancy week calculator, or your date with the due date calculator.
Your baby this week
The NHS gives a length of around 26.7cm from head to toe and compares it to a carrot. Your baby is now heavier than the placenta, which keeps growing to do its work.
- Hair. A downy layer of lanugo, which probably helps keep your baby at the right temperature and usually disappears before birth, plus hair and eyebrows (NHS).
- Hearing. Your baby can now hear noises and voices from outside the womb (NHS).
- Movement and blood. Cleveland Clinic says limb movements are coordinated and frequent, and bone marrow now helps make blood cells.
- Skills for later. The NHS says your baby is developing sucking and practising breathing. ACOG's description of weeks 21 to 24 adds that fat is forming under the skin.
Your body and common symptoms
Feeling unsteady. Your centre of gravity has shifted and your joints are looser (NHS). Flat, supportive shoes and a free hand on stairs help. If you travel on public transport, a "baby on board" badge can prompt people to offer a seat.
Sleep. Your baby may be wide awake when you are ready to sleep. Nap when you can (NHS).
Symptoms the NHS lists this week: round ligament pains on the sides of your bump, piles, backache, leg cramps, and sore or leaky breasts.
Food. The NHS encourages five portions of fruit and veg a day and wholegrain, lower-sugar choices when you have them. Homemade versions of takeaway favourites are an easy swap.
Is this normal? When to call
Unsteady moments, a baby who wakes up at bedtime and achy breasts are all normal at 21 weeks.
Tell your midwife or doctor about any fall, even if you feel fine. Call straight away for vaginal bleeding, fluid leaking, tummy pain or regular tightenings, a severe headache, vision changes, sudden swelling, or a temperature of 38C (100.4F) or above. If you have felt movements and they slow down or stop, call immediately, day or night. In the UK, call your maternity unit, or NHS 111 if you cannot reach them; 999 in an emergency. In the US, call your OB-GYN or midwife, or 911 in an emergency.
If your scan raised questions
For some families, the mid-pregnancy scan brings unexpected news. The NHS suggests talking your options through with your midwife or doctor, and mentions the free helpline run by the charity Antenatal Results and Choices in the UK. You do not have to decide anything in a hurry; it is fine to ask for another appointment to go over the results.
Appointments and tests around 21 weeks
- UK: if your 20-week scan has not happened, tell your midwife: it can sometimes be done up to 23 weeks. Not had the whooping cough vaccine yet? The NHS offers it from 16 to 32 weeks. Your next routine check is at 25 weeks if this is your first baby.
- US: if your anatomy ultrasound is not done yet, it is usually at 18 to 22 weeks (ACOG). Your provider will check your blood pressure and urine at routine visits.
Your checklist for week 21
For your partner
The NHS suggests partners join antenatal classes, even if you have had children before. Your voice reaches the bump now, so a bedtime chat counts.
Previous and next week
Week 20: halfway · Week 22: stretch marks, piles and lungs
Read the full second trimester guide or see all pregnancy weeks.
Questions parents ask
How many months is 21 weeks pregnant?
Cleveland Clinic groups weeks 21 to 24 as month six, so 21 weeks is the start of the sixth month.
I fell over at 21 weeks. What should I do?
The NHS says your baby is well protected, so try not to panic, but let your midwife know as soon as possible. Call straight away if you have bleeding, fluid leaking, tummy pain or tightenings, or notice a change in your baby's movements.
Why is my baby so active when I lie down?
The NHS explains that your baby is establishing waking and sleeping patterns, and may be wide awake just as you want to sleep. Daytime naps can help make up for lost sleep.
Sources
- NHS — Week-by-week guide to pregnancy, week 21 checked October 13, 2026
- Cleveland Clinic — Fetal Development, week-by-week stages checked October 13, 2026
- ACOG — How Your Fetus Grows During Pregnancy checked October 13, 2026
- NHS — Whooping cough vaccine checked October 13, 2026
- NHS — Your baby's movements checked October 13, 2026
- NHS — Your antenatal appointments checked October 13, 2026
- ACOG — Ultrasound Exams checked October 13, 2026
Mamavela explains; it does not replace your midwife, doctor or pediatrician, who know your situation. If something worries you, call them. In an emergency, call 911 (US), 999 (UK) or 112.