16 weeks pregnant: development and the 16-week appointment
16 weeks pregnant is a stage at which, the NHS says, the baby is around 11.6 cm long from head to bottom, about the size of an avocado, and the NHS lists a midwife appointment around now (NHS, 2026). This page sets out what the NHS, ACOG and the Office on Women’s Health say about week 16, including sizes that differ between the NHS and the Office on Women’s Health.
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Medically reviewed by Dr Mary Davidson, Reproductive endocrinologist, checked on
Seek help now if you have any of these symptoms
Bleeding and cramping are common in early pregnancy and often do not mean miscarriage. The symptoms below, however, need to be assessed straight away.
- Soaking two or more sanitary pads per hour, or passing large clots
- Severe lower abdominal pain, especially on one side
- Pain at the tip of the shoulder
- Dizziness, fainting, or collapse
- Signs of shock — pale, clammy, rapid heartbeat, breathlessness
- Pain on opening bowels, or bladder or bowel changes
- Feeling very unwell, or feverish
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How the sources describe development around 16 weeks
- Size and shape, NHS
- The NHS week 16 page in its week-by-week guide (NHS, 2026) gives the baby as around 11.6 cm long from head to bottom and compares it to an avocado. It says the baby is starting to pull faces, though any smiling or frowning is random because there is no muscle control yet, and that the hands can form fists.
- Size and development, Office on Women’s Health
- The Office on Women’s Health page on the stages of pregnancy (2025) says that at 16 weeks muscle tissue and bone continue to form, creating a more complete skeleton, the skin begins to form and can nearly be seen through, meconium develops in the intestinal tract, the baby makes sucking motions with the mouth, and the baby is about 4 to 5 inches long and weighs almost 3 ounces. The NHS and the Office on Women’s Health each give their own size, and this page does not convert one into the other.
- Development, ACOG
- ACOG’s list for weeks 13 to 16 includes bones hardening, especially the long bones, thin see-through skin that will start to thicken soon, a defined neck, and lungs beginning to form tissue for exchanging oxygen and carbon dioxide (ACOG, How Your Fetus Grows During Pregnancy, 2024).
Weeks are counted from the first day of the last menstrual period, so about two weeks of the count come before conception (ACOG, How Your Fetus Grows During Pregnancy, 2024). This page covers 16 weeks 0 days to 16 weeks 6 days.
A normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, so at 16 weeks about 24 weeks remain to the estimated due date (ACOG, How Your Fetus Grows During Pregnancy, 2024). Your own due date depends on your last period or a scan, not on the week alone.
The 16-week appointment
The NHS page on antenatal appointments says that at 16 weeks the midwife asks about your health and wellbeing and answers any questions, discusses the results of previous screening tests, gives you information about the next scan you will be offered, and asks to check your blood pressure and test your urine (NHS, "Antenatal (pregnancy) care and appointments", 2026). The NHS week 16 page says you will also get the results of any blood tests from your booking appointment, that you will probably have been offered a test for HIV, hepatitis B and syphilis, and that the urine is checked for protein, which can be a sign of the condition pre-eclampsia (NHS, "Week 16 of pregnancy", 2026).
In the US, the Office on Women’s Health describes a maternal serum screen, a blood test done at 15 to 20 weeks, to detect a higher risk of certain conditions (Office on Women’s Health, Prenatal care and tests, undated). Screening is offered by your care team and it is your choice.
Whooping cough vaccination
The NHS says its recommendation is that all pregnant women have the whooping cough vaccination, that it is usually offered around week 20, and that it can be given from week 16 to week 32 (NHS, "Whooping cough vaccination in pregnancy", 2023). In the US, the CDC recommends the Tdap vaccine during the 27th through 36th week of each pregnancy, preferably in the earlier part of that time (CDC, Tdap Vaccination for Pregnant Women, 2025). The UK and US windows differ, so ask your midwife or doctor which applies to you.
Work out which week you are in
The Pregnancy week calculator takes your last period, conception date, scan or IVF transfer and shows how many weeks and days pregnant you are today.
Sources
- NHS Best Start in Life. Week 16 of pregnancy (week-by-week guide to pregnancy). nhs.uk, 2026.
- NHS. Antenatal (pregnancy) care and appointments. nhs.uk, 2026.
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy (FAQ156). ACOG, 2024.
- Office on Women's Health, US Department of Health and Human Services. Stages of pregnancy. womenshealth.gov, 2025.
- Office on Women's Health, US Department of Health and Human Services. Prenatal care and tests. womenshealth.gov.
- Centers for Disease Control and Prevention. Tdap Vaccination for Pregnant Women. CDC, 2025.
- NHS. Whooping cough vaccination in pregnancy. nhs.uk, 2023.
Frequently asked questions
What happens at the 16-week appointment?
The NHS says the midwife asks about your health and wellbeing, discusses the results of previous screening tests, gives you information about the next scan, and asks to check your blood pressure and test your urine (NHS, 2026).
When is the whooping cough vaccine given in pregnancy?
The NHS says it can be given from week 16 to week 32 and is usually offered around week 20 (NHS, 2026). The CDC recommends it during the 27th through 36th week (CDC, 2025). These are the schedules of two different countries.