13 weeks pregnant: where the second trimester starts
13 weeks pregnant is a stage at which, the NHS says, you are a third of the way through your pregnancy and the baby is around 7.4 cm long, about the size of a peach (NHS, 2026). This page sets out what the NHS, ACOG and the Office on Women’s Health say about week 13, and where they place the start of the second trimester.
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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 13 weeks
- Size and shape, NHS
- The NHS week 13 page in its week-by-week guide (NHS, 2026) gives the baby as around 7.4 cm long and compares it to a peach. It says the ovaries or testes are fully developed inside and final tweaks are being made on the outside.
- Movement, NHS
- The NHS says the baby is moving around, at first in jerky, random movements and then in ones that look more deliberate, and that you may not feel any movement until around week 17. It says some babies suck their thumb in the womb, which helps to develop the sucking reflex they will need for feeding (NHS, 2026).
- Development, ACOG
- ACOG groups weeks 13 to 16 together. Its list includes the bones hardening, especially the long bones, thin see-through skin that will start to thicken soon, toenails forming, a defined neck and developed lower limbs, and hearing starting to develop (ACOG, How Your Fetus Grows During Pregnancy, 2024). The list is not broken down week by week.
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 13 weeks 0 days to 13 weeks 6 days.
A normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, so at 13 weeks about 27 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.
Which trimester is week 13?
The NHS week-by-week guide begins its second trimester pages at week 13 (NHS, 2026), and the Office on Women’s Health defines the second trimester as week 13 to week 28 (Office on Women’s Health, Stages of pregnancy, 2025). ACOG defines the first trimester as running to 13 weeks and 6 days, with the second starting at 14 weeks and 0 days (ACOG, How Your Fetus Grows During Pregnancy, 2024). Under ACOG’s definition all of week 13 is still in the first trimester, and under the NHS’s it is in the second. The hub for pregnancy trimesters sets the three side by side, and the guide to the second trimester covers the weeks ahead.
Your body at 13 weeks, as the NHS describes it
The NHS says a small baby bump may now be visible as the uterus grows upwards and outwards, and that the urge to pee more often should stop as the womb moves away from the bladder. It says there is a lot more blood pumping around the pelvic area, which some women find increases their sex drive, and that it can also make you feel more thirsty (NHS, 2026).
The NHS also says that swollen, painful and bleeding gums are common during pregnancy (NHS, 2026). This page cannot say whether bleeding from any part of the body is expected for you. For vaginal bleeding, use the box above.
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 13 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
Is 13 weeks pregnant the first or second trimester?
It depends on the source. The NHS week-by-week guide and the Office on Women’s Health put week 13 in the second trimester (NHS, 2026; Office on Women’s Health, 2025). ACOG puts it at the end of the first, which runs to 13 weeks and 6 days (ACOG, 2024).
Can I feel the baby move at 13 weeks?
The NHS says you may not feel any movement until around week 17 (NHS, 2026). Its week 17 page says many women spot the first signs of movement between 18 and 24 weeks.