Free family guide · Pregnancy week by week · Second trimester

21 weeks pregnant

Week 21 is in the second trimester in both the US Office on Women's Health (OWH) system and the UK's NHS week-by-week guide.[1][2]

Last reviewed: Next review: No outside ads and no affiliate links.

Sources on this page: OWH; NHS; ACOG; MedlinePlus; WHO; NICHD; CDC; NIMH; HRSA

This page shares what health organizations say. It is not medical advice. It is general information. Talk to your doctor, midwife or pediatrician about your own situation.

In an emergency, call 911.

At a glance

  • The NHS says the baby is around 26.7 cm long from head to heel.[3]
  • The NHS says the baby is now heavier than the placenta.[3]
  • The NHS suggests eating 5 portions of fruit and vegetables a day.[3]

Which trimester is week 21?

OWH counts the trimesters as weeks 1 to 12, 13 to 28 and 29 to 40.[1] The NHS guide counts them as weeks 4 to 12, 13 to 27 and 28 to 41.[2] ACOG counts the first trimester to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks to 40 weeks 6 days.[4]

All three place week 21 in the second trimester.[1][2][4]

Your body this week

The NHS lists these symptoms around week 21:[3]

  • pain in the ligaments around your belly;[3]
  • piles;[3]
  • backache;[3]
  • leg cramps;[3]
  • sore or leaky breasts.[3]

The NHS says to tell your midwife if you have a fall.[3]

Your baby this week

The NHS says the baby is around 26.7 cm long from head to heel.[3]

The NHS says the baby is now heavier than the placenta.[3] It says the baby can hear noises and voices outside the womb, and waking and sleeping patterns are forming.[3]

Visits and tests around week 21

Sources differ

Sources give different weeks for amniocentesis:[5][6][7]

  • OWH lists amniocentesis at weeks 14 to 20.[5]
  • MedlinePlus lists it between weeks 14 and 20.[6]
  • ACOG says it is usually done at 15 to 20 weeks.[7]

Sources differ

Sources give different weeks for the maternal serum (quad) screen:[5][6][8]

  • OWH lists the maternal serum screen at weeks 15 to 20.[5]
  • MedlinePlus lists the quad screen between weeks 15 and 22, most accurate at weeks 16 to 18.[6]
  • ACOG says the quad screen is done at 15 to 22 weeks.[8]

Sources differ

Sources give different weeks for the mid-pregnancy ultrasound (anatomy scan):[5][6][9][10][11]

  • OWH lists a standard ultrasound, often at weeks 18 to 20.[5]
  • MedlinePlus lists an anatomy ultrasound around week 20.[6]
  • ACOG says at least one standard exam is done, usually at 18 to 22 weeks.[9]
  • The NHS says its anomaly scan is offered at around 18 to 20 weeks.[10]
  • The World Health Organization (WHO) recommends one ultrasound before 24 weeks.[11]

MedlinePlus says prenatal testing is optional and is your decision, made with your provider.[12]

For a healthy pregnancy, NICHD describes visits once a month until 28 weeks.[13]

Things to do this week

  • The NHS suggests eating 5 portions of fruit and vegetables a day.[3]
  • The NHS says to talk to your midwife if the anomaly scan brings unexpected news.[3]
  • CDC says to avoid lying flat on your back during exercise after the first trimester.[14]
  • MedlinePlus lists 27 mg of iron and 600 mcg of folic acid a day during pregnancy.[15]

Feelings check-in

The National Institute of Mental Health (NIMH) says perinatal depression includes depression during pregnancy, and its signs last at least 2 weeks.[16]

HRSA says the National Maternal Mental Health Hotline, 1-833-852-6262, takes calls and texts 24/7 in English and Spanish.[17]

When to call your provider or get help

MedlinePlus says gestational high blood pressure and preeclampsia start after 20 weeks, and high blood pressure is often found only at checkups.[18]

CDC lists urgent warning signs to watch for during pregnancy and in the year after birth, including:[19]

  • a headache that will not go away or gets worse;[19]
  • changes in vision;[19]
  • extreme swelling of the hands or face;[19]
  • severe belly pain that does not go away;[19]
  • the baby's movement stopping or slowing;[19]
  • vaginal bleeding or leaking fluid;[19]
  • thoughts of harming yourself or the baby.[19]

Questions families ask

Which trimester is week 21?

OWH places week 21 in the second trimester.[1] The NHS guide places it in the second trimester.[2] ACOG places it in the second trimester.[4]

How big is the baby at 21 weeks?

The NHS says the baby is around 26.7 cm long from head to heel.[3]

What if the scan shows something unexpected?

The NHS says to talk to your midwife.[3] MedlinePlus says an abnormal screening result means more information is needed, and a diagnostic test confirms whether a condition is present.[12]

Free from UpOnly Learning. Written for families of every faith. No outside ads and no affiliate links. Every fact on this page comes from the sources listed below. How we source and review these pages

Updated 2026-10-24: first published.

Sources

All sources were opened and checked on the date shown.

  1. Office on Women's Health (HHS). Stages of pregnancy. Checked October 24, 2026.
  2. NHS (United Kingdom). Week-by-week guide to pregnancy. Checked October 24, 2026.
  3. NHS (United Kingdom). 21 weeks pregnant guide. Checked October 24, 2026.
  4. American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy. Checked October 24, 2026.
  5. Office on Women's Health (HHS). Prenatal care and tests. Checked October 24, 2026.
  6. MedlinePlus (National Library of Medicine). Prenatal care in your second trimester. Checked October 24, 2026.
  7. American College of Obstetricians and Gynecologists. Prenatal Genetic Diagnostic Tests. Checked October 24, 2026.
  8. American College of Obstetricians and Gynecologists. Prenatal Genetic Screening Tests. Checked October 24, 2026.
  9. American College of Obstetricians and Gynecologists. Ultrasound Exams. Checked October 24, 2026.
  10. NHS (United Kingdom). 18 weeks pregnant guide. Checked October 24, 2026.
  11. World Health Organization. New guidelines on antenatal care for a positive pregnancy experience (news release, Nov 7, 2016). Checked October 24, 2026.
  12. MedlinePlus (National Library of Medicine). Prenatal Testing. Checked October 24, 2026.
  13. Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH). What happens during prenatal visits?. Checked October 24, 2026.
  14. Centers for Disease Control and Prevention. Pregnant & Postpartum Activity: An Overview. Checked October 24, 2026.
  15. MedlinePlus (National Library of Medicine). Pregnancy and Nutrition. Checked October 24, 2026.
  16. National Institute of Mental Health (NIH). Perinatal Depression. Checked October 24, 2026.
  17. Health Resources and Services Administration (HRSA). National Maternal Mental Health Hotline. Checked October 24, 2026.
  18. MedlinePlus (National Library of Medicine). High Blood Pressure in Pregnancy. Checked October 24, 2026.
  19. Centers for Disease Control and Prevention (HEAR HER campaign). Urgent Maternal Warning Signs and Symptoms. Checked October 24, 2026.