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Second trimester

21 weeks pregnant

At 21 weeks you are in the second half of pregnancy. Your baby moves a lot, is settling into waking and sleeping patterns, and can hear noises and voices from outside the womb. If you've had your anatomy scan, your provider can talk you through the results.

This page covers 21 weeks 0 days to 21 weeks 6 days (21+0 to 21+6), counted from the first day of the last period. Check how far along you are

Enter it to see the calendar dates of your week 21.

Botanical illustration of an eggplant

About the size of an eggplant

Typical estimated weight 400 g (14 oz) at 21+0, about 465 g (1 lb 0 oz) by 21+6. This is the 50th centile; healthy babies commonly vary by about 12.7% either way.1

A reference value, not a measurement of your baby.

How your baby is developing

  • Your baby can now hear noises and voices from outside the womb.2,3
  • Limb movements are becoming coordinated and frequent, and your baby is setting up waking and sleeping patterns. They may be wide awake just as you are ready to sleep.2,4
  • Your baby can swallow and is building skills such as sucking and breathing movements.2,3
  • Your baby is now heavier than the placenta, which keeps growing as it supplies food and removes waste.2
  • Hair and eyebrows are developing, along with lanugo, a fine body hair that usually disappears before birth.2
  • Sometime between about 21 and 25 weeks, the bone marrow begins making blood cells.3,4

Changes you may notice

Everyone’s experience is different. You may notice some of these, or none.

  • Your center of gravity has changed and your joints are looser, so you may feel off balance. If you fall, your baby is well protected, but tell your midwife or provider as soon as possible.2
  • Piles, backache, heartburn, bloating, constipation, leg cramps and swollen hands and feet are all possible. Not everyone has them. Seek medical advice for any vaginal bleeding.2
  • If your baby is active when you're trying to rest, a daytime nap may help make up for lost sleep.2

Care around this week

  • If your anatomy scan brought unexpected news, your midwife or doctor can talk through your options. In the UK, the charity Antenatal Results and Choices runs a free helpline. If you haven't had the scan yet, it is usually done by 22 weeks in the US.2,5
  • If your baby's movements slow down, change or stop, contact your midwife or maternity unit straight away, even in the middle of the night. There is no set number of movements to count; what matters is your baby's usual pattern.6
  • UK: if you haven't had the whooping cough vaccine, talk to your midwife or GP. It can be given from 16 weeks, ideally before 32. In the US, Tdap is recommended during weeks 27 through 36.2,7,8

Questions to ask your provider

  • Can you walk me through my anatomy scan results?
  • Do I need any follow-up scans?
  • What can I do about backache or piles?
  • If I have a fall, who should I call?
When to contact your provider straight away

Call 911 (US) or 999 (UK) in an emergency, such as heavy bleeding or bleeding with severe pain, a seizure, chest pain, or severe trouble breathing.9,10,11,12

These symptoms are not always serious, but each one needs prompt advice from your provider, midwife or maternity unit (or NHS 111 in the UK if you cannot reach them) rather than waiting for your next appointment, and it helps to tell them you are pregnant.

  • Vaginal bleeding or spotting at any stage of pregnancy. Call 911 or 999 if the bleeding is heavy or comes with severe belly pain, shoulder pain, or feeling faint or dizzy.11,13
  • Fluid leaking or gushing from your vagina, or discharge that is unusual for you. Get checked straight away if the fluid is greenish, colored or smells bad.11,12,14,15
  • Your baby moving less than usual, not moving, or a change in their usual pattern of movements. Call immediately, even in the middle of the night, and do not use a home Doppler to check yourself.6,11
  • A severe headache that does not go away with simple painkillers or gets worse, or sudden changes in your vision such as blurring or seeing flashing lights.9,11,12
  • Sudden swelling of your face, hands or feet, or swelling, redness or pain in a leg.9,11,12
  • Belly pain that is severe or does not go away after resting, or pain just below the ribs.9,11,16
  • Regular tightenings or contractions, period-type pains, or backache that is unusual for you before 37 weeks, which can be signs of preterm labor.15,16
  • A fever of 100.4°F (38°C) or higher, or feeling hot, cold or shivery.11,12,15
  • Shortness of breath that comes on suddenly or happens when you lie flat, chest pain, a racing or pounding heart, or dizziness or fainting.11,12
  • Very itchy skin, especially on the palms of your hands or soles of your feet, or itching that is worse at night.17
  • Being sick so often that you cannot keep food or drink down, or signs of dehydration such as dark urine, peeing very little, or feeling dizzy.11,18
  • Thoughts of harming yourself or your baby. Get help right away, and call 911 or 999 if you think you might act on them.11,19

Questions about 21 weeks

Can my baby hear my voice at 21 weeks?

Yes. By now your baby can hear noises and voices from outside the womb.2

What should I do if my baby's movements change?

Contact your midwife or maternity unit straight away if your baby's movements slow down, change or stop. Don't wait until the next day or your next appointment, and don't use a home doppler to check on your baby.6

Is it normal for my baby to be more active at night?

Yes. Babies develop their own waking and sleeping patterns, and yours may be active when you're trying to rest.2

Sources

  1. In utero analysis of fetal growth: a sonographic weight standardRadiology 181(1):129–133, 1991
  2. 21 weeks pregnant guideNHS Best Start in Life
  3. Fetal developmentUS National Library of Medicine
  4. Fetal development: week-by-week stages of pregnancyCleveland Clinic
  5. Ultrasound ExamsAmerican College of Obstetricians and Gynecologists
  6. Your baby's movementsNHS (England)
  7. Vaccinations in pregnancyNHS (England)
  8. Tdap Vaccination for Pregnant WomenUS Centers for Disease Control and Prevention
  9. Pre-eclampsiaNHS (England)
  10. Epilepsy and pregnancyNHS (England)
  11. Urgent Maternal Warning Signs and Symptoms (Hear Her campaign)US Centers for Disease Control and Prevention
  12. Pregnancy symptoms you need to get help forNHS (England)
  13. Vaginal bleeding in pregnancyNHS (England)
  14. Signs of going into labourNHS Best Start in Life
  15. Premature labour and birthNHS (England)
  16. Stomach pain in pregnancyNHS (England)
  17. Itching in pregnancyNHS (England)
  18. Severe vomiting in pregnancyNHS (England)
  19. Depression in pregnancyNHS (England)

Links checked November 12, 2026. Citing a source does not mean it endorses this site.

Written from the sources above and checked against them on 2026-11-12. Not reviewed by a clinician. Our editorial policy