12 weeks pregnant: where the first trimester ends
12 weeks pregnant is a stage at which, the NHS says, the baby is 5.4 cm long from head to bottom, about the size of a plum, and the first trimester is coming to a close (NHS, 2026). This page sets out what the NHS, ACOG and the Office on Women’s Health say about week 12, including where each draws the trimester line.
Last updated
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
Showing routes for United States.
How the sources describe development around 12 weeks
- Size and shape, NHS
- The NHS week 12 page in its week-by-week guide (NHS, 2026) gives the baby as 5.4 cm long from head to bottom and compares it to a plum. It says the internal organs and muscles have grown, the heartbeat can be picked up on an ultrasound scan, the skeleton is made of tissue hardening into bone, and the sex organs have formed, although most scans will not be able to tell the sex until later.
- Size and development, Office on Women’s Health
- The Office on Women’s Health page on the stages of pregnancy (2025) says that at 12 weeks the nerves and muscles begin to work together and the baby can make a fist, the external sex organs show if the baby is a boy or a girl, the eyelids close to protect the developing eyes and will not open again until the 28th week, and the baby is about 3 inches long and weighs almost an ounce. The NHS and the Office on Women’s Health give different sizes for week 12, and this page gives each as its source states it.
- Development, ACOG
- ACOG’s list for weeks 9 to 12 includes eyelids forming but remaining closed, the pancreas starting to make insulin and fingernails forming (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 12 weeks 0 days to 12 weeks 6 days.
A normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, so at 12 weeks about 28 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.
Where the first trimester ends: three answers
- The NHS week-by-week guide treats weeks 4 to 12 as the first trimester, and begins its second trimester pages at week 13 (NHS, 2026).
- The Office on Women’s Health defines the first trimester as week 1 to week 12 (Office on Women’s Health, 2025).
- ACOG defines the first trimester as running from the first day of the last menstrual period to 13 weeks and 6 days, and the second trimester as 14 weeks and 0 days to 27 weeks and 6 days (ACOG, 2024).
The trimester lines are conventions and the three bodies draw them differently, so a person can be in the first trimester by one and in the second by another. The hub for pregnancy trimesters sets the three side by side.
Your body at 12 weeks, as the NHS describes it
The NHS says there is a good chance that the placenta is now feeding the baby, having taken over from the yolk sac, and that once this happens your hormones will calm down a bit. It says your waist is probably thickening and your breasts getting bigger, and that as sickness subsides you may start to feel hungrier (NHS, 2026).
The NHS says that if you wonder whether your bump should look bigger or smaller, many factors decide how big your stomach gets, including your hormones, your weight before pregnancy, how many babies you have already had and the strength of your muscles (NHS, 2026).
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 12 of pregnancy (week-by-week guide to pregnancy). 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.
Frequently asked questions
Which trimester is 12 weeks pregnant?
It depends on the source. The NHS week-by-week guide and the Office on Women’s Health treat week 12 as the last week of the first trimester (NHS, 2026; Office on Women’s Health, 2025). ACOG’s first trimester runs to 13 weeks and 6 days, so week 12 is also in the first trimester by that definition (ACOG, 2024). The sources differ on when the second trimester starts, not on week 12 itself.
How big is the baby at 12 weeks pregnant?
The NHS says 5.4 cm long from head to bottom, about the size of a plum (NHS, 2026). The Office on Women’s Health says about 3 inches long and almost an ounce (Office on Women’s Health, 2025). The two bodies give different figures.