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21 Weeks Pregnant: Baby Size, Symptoms and What to Do

Week 21 is in the second trimester, month 5 of pregnancy. Here's how big your baby is, what's developing, what you may feel and what to do this week.

Week 21 · 2nd trimester · month 5
About the size of a carrot
about 10.6 in (27 cm) · 14 oz (398 g) (head to heel)

Your baby this week

Your baby now outweighs the placenta. Sounds from outside, including your voice, are reaching your baby.

At 21 weeks your baby weighs more than the placenta, and growth is speeding up. The digestive system is practicing: it absorbs small amounts of sugar from swallowed amniotic fluid. Bone marrow is taking on more of the work of making blood cells. Movements are becoming more regular, and a partner may be able to feel them with a hand on your belly. Stretch marks may appear, and the skin on your belly can feel tight or itchy. Moisturizing can ease the tightness, though it may not prevent stretch marks. Intense itching, especially on the palms and soles and worse at night, can be a sign of a liver condition of pregnancy (cholestasis), so always tell your provider.

By the numbers

Week 21 covers days 147 to 153 counted from the first day of your last period, which is 53% of a 40-week pregnancy. About 133 days remain until a 40-week due date. Since last week, an average baby gains about 2.4 oz (68 g). By date of conception, your baby is about 19 weeks old.

Pregnancy is dated from the first day of your last period, so conception happens around week 2. Your due date is 280 days (40 weeks) from that day, and an early ultrasound may adjust it.

Your body

Stretch marks may show up on your belly, breasts or thighs. Moisturizing can ease the tight, itchy feeling.

Tests and to-dos this week

Talking, reading or singing to your baby is a sweet way to bond. Plan ahead: the glucose screening test is usually done between 24 and 28 weeks.

  • Quad screen (optional) · weeks 15–22 · test

  • Anatomy ultrasound · weeks 18–22 · test

Coming up: Glucose screening for gestational diabetes (from week 24). Full prenatal visit schedule

Tips for the second trimester

  • Most healthy pregnant people are advised to get about 150 minutes of moderate activity a week, such as walking or swimming.
  • Your recommended weight gain depends on your pre-pregnancy BMI, and gain usually speeds up in the second trimester.
  • Once you feel your baby regularly, learn their usual pattern; a change in pattern matters more than any count.
  • The flu shot is recommended at any point in pregnancy during flu season.

Call your provider or labor and delivery right away if you have:

  • vaginal bleeding or leaking fluid
  • regular contractions or pelvic pressure before 37 weeks
  • a severe headache, vision changes, or sudden swelling of the face or hands
  • a fever of 100.4 °F (38 °C) or higher, or painful urination
  • after 24 weeks, your baby moving less or differently

In an emergency, call 911.

Last week and next week

Last week (20): From this week, length is measured from head to heel, which is why the number jumps.

Next week (22): The lungs are developing, and your baby practices breathing movements.

Frequently asked questions

How big is the baby at 21 weeks?

About the size of a carrot: about 10.6 in (27 cm) · 14 oz (398 g), measured head to heel. These are rounded averages, and every baby grows at their own pace.

How many months is 21 weeks pregnant?

21 weeks is in month 5 and the second trimester. About 19 weeks (133 days) remain until a 40-week due date. Months are approximate; providers count weeks and days.

Is itching in pregnancy dangerous?

Mild itching from stretching skin is common. Intense itching that starts on the palms and soles and is worse at night can signal cholestasis of pregnancy, which is checked with a blood test, so always report it.

What's the next test or appointment after week 21?

Next up: Glucose screening for gestational diabetes, from week 24. Your provider may schedule things differently depending on your health and local practice.

Sources

Last reviewed: November 14, 2026 · Kitpebble editorial team

This page is for general information. It isn't medical advice or a diagnosis and doesn't replace your OB-GYN or midwife. Sizes are rounded averages. Our editorial policy

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