Free family guide · Pregnancy week by week · Second trimester

18 weeks pregnant

Week 18 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]

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Sources on this page: OWH; NHS; CDC; ACOG; MedlinePlus; WHO; NICHD; FDA; 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 14.2 cm long.[3]
  • The NHS says the baby's hearing, swallowing and sucking reflexes are developing.[3]
  • CDC says to avoid lying flat on your back during exercise after the first trimester.[4]

Which trimester is week 18?

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.[5]

All three place week 18 in the second trimester.[1][2][5]

Your body this week

The NHS says your blood pressure may be lower, so stand up slowly.[3] It says a dark line down your belly, called linea nigra, is normal.[3]

Your baby this week

The NHS says the baby is around 14.2 cm long.[3]

The NHS says the baby's hearing, swallowing and sucking reflexes are developing.[3]

Visits and tests around week 18

The NHS says the anomaly scan looks at the baby's development in detail.[3] It says the scan finds a range of conditions, but not all of them.[3]

Sources differ

Sources give different weeks for amniocentesis:[6][7][8]

  • OWH lists amniocentesis at weeks 14 to 20.[6]
  • MedlinePlus lists it between weeks 14 and 20.[7]
  • ACOG says it is usually done at 15 to 20 weeks.[8]

Sources differ

Sources give different weeks for the maternal serum (quad) screen:[6][7][9]

  • OWH lists the maternal serum screen at weeks 15 to 20.[6]
  • MedlinePlus lists the quad screen between weeks 15 and 22, most accurate at weeks 16 to 18.[7]
  • ACOG says the quad screen is done at 15 to 22 weeks.[9]

Sources differ

Sources give different weeks for the mid-pregnancy ultrasound (anatomy scan):[6][7][10][3][11]

  • OWH lists a standard ultrasound, often at weeks 18 to 20.[6]
  • MedlinePlus lists an anatomy ultrasound around week 20.[7]
  • ACOG says at least one standard exam is done, usually at 18 to 22 weeks.[10]
  • The NHS says its anomaly scan is offered at around 18 to 20 weeks.[3]
  • 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

  • CDC says to avoid lying flat on your back during exercise after the first trimester.[4]
  • MedlinePlus lists 27 mg of iron and 600 mcg of folic acid a day during pregnancy.[14]
  • The FDA advises 2 to 3 servings a week (4 ounces each) of fish from its Best Choices list, such as salmon, sardines, shrimp, cod, tilapia and canned light tuna.[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

CDC lists urgent warning signs to watch for during pregnancy and in the year after birth, including:[18]

  • a headache that will not go away or gets worse;[18]
  • dizziness or fainting;[18]
  • a fever of 100.4°F (38°C) or higher;[18]
  • severe belly pain that does not go away;[18]
  • vaginal bleeding or leaking fluid;[18]
  • thoughts of harming yourself or the baby.[18]

Questions families ask

Which trimester is week 18?

OWH places week 18 in the second trimester.[1] The NHS guide places it in the second trimester.[2] ACOG places it in the second trimester.[5]

How big is the baby at 18 weeks?

The NHS says the baby is around 14.2 cm long.[3]

What does the anatomy scan check?

ACOG says the standard exam checks the baby's development, screens for major problems, and looks at position, heart rate, size, fluid and the placenta.[10] The NHS says the scan finds a range of conditions, but not all of them.[3]

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). 18 weeks pregnant guide. Checked October 24, 2026.
  4. Centers for Disease Control and Prevention. Pregnant & Postpartum Activity: An Overview. Checked October 24, 2026.
  5. American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy. Checked October 24, 2026.
  6. Office on Women's Health (HHS). Prenatal care and tests. Checked October 24, 2026.
  7. MedlinePlus (National Library of Medicine). Prenatal care in your second trimester. Checked October 24, 2026.
  8. American College of Obstetricians and Gynecologists. Prenatal Genetic Diagnostic Tests. Checked October 24, 2026.
  9. American College of Obstetricians and Gynecologists. Prenatal Genetic Screening Tests. Checked October 24, 2026.
  10. American College of Obstetricians and Gynecologists. Ultrasound Exams. 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. MedlinePlus (National Library of Medicine). Pregnancy and Nutrition. Checked October 24, 2026.
  15. US Food and Drug Administration. Advice about Eating Fish. 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. Centers for Disease Control and Prevention (HEAR HER campaign). Urgent Maternal Warning Signs and Symptoms. Checked October 24, 2026.