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

Week 18 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 18 · 2nd trimester · month 5
About the size of a bell pepper
about 5.5 in (14 cm) · 7.8 oz (222 g) (crown to rump)

Your baby this week

Hearing and touch are developing, and your baby is practicing swallowing and sucking. There's a lot of wriggling and rolling going on.

Week 18 opens the window for the anatomy scan, which usually happens between 18 and 22 weeks. The sonographer looks at your baby's brain, face, spine, heart, abdominal wall, kidneys and limbs, checks growth, and notes where the placenta is and how much amniotic fluid there is. It can take 30 to 60 minutes, and if your baby's position makes some views hard, you may need to come back. Your baby can yawn and hiccup now, and the nervous system is maturing quickly: the protective myelin coating around nerves is starting to form. It's a good week to think about what you'd like to learn at the scan, including whether you want to know the baby's sex.

By the numbers

Week 18 covers days 126 to 132 counted from the first day of your last period, which is 45% of a 40-week pregnancy. About 154 days remain until a 40-week due date. Since last week, an average baby gains about 1.5 oz (43 g). By date of conception, your baby is about 16 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

Backaches may start as your center of gravity shifts. Low-heeled shoes, good posture and lifting with your knees can help. Call your provider if back pain is severe or comes with fever, burning when you pee, or bleeding.

Tests and to-dos this week

The anatomy ultrasound, usually done at 18 to 22 weeks, checks your baby's organs, growth and the placenta. If you want to know the sex, you can often find out at this scan, or ask them to keep it a surprise.

  • 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 (17): Your baby's one-of-a-kind fingerprints are forming, and the nails keep growing.

Next week (19): Behind the buds for baby teeth, buds for permanent teeth are now forming.

Frequently asked questions

How big is the baby at 18 weeks?

About the size of a bell pepper: about 5.5 in (14 cm) · 7.8 oz (222 g), measured crown to rump. These are rounded averages, and every baby grows at their own pace.

How many months is 18 weeks pregnant?

18 weeks is in month 5 and the second trimester. About 22 weeks (154 days) remain until a 40-week due date. Months are approximate; providers count weeks and days.

What do they check at the anatomy scan?

Your baby's anatomy (brain, face, spine, heart, kidneys, abdominal wall, limbs), growth, where the placenta sits and the amount of amniotic fluid. If a view isn't clear, you may be asked to come back.

What's the next test or appointment after week 18?

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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