First trimester: weeks 1 to 13

The stage of the fastest change, from before conception to a tiny, fully formed fetus about 7 cm long. It’s often the most tiring part of pregnancy too.

Checked against the sources listed on this page. Not reviewed by a clinician. Last updated 5 November 2026. How we check.

When is the first trimester?

This site counts it as weeks 1 to 13, ending at 13 weeks 6 days, counted from the first day of your last period. Some guides, including the NHS week-by-week guide, move on to the second trimester at week 13, so you’ll see both.

How your baby develops

There’s no baby in the first two weeks, because conception usually happens around the end of week 2. The NHS gives a size of about 2 mm at weeks 4 and 5, 16 mm at week 8, and 5.4 cm from head to bottom at week 12.

By weeks 6 to 7, the heart beats with a regular rhythm. Tissue that will become the spine and other bones is growing, and by week 12 the baby has reflexes and curling fingers. [Follow it week by week below](#weeks).

Common symptoms

Everyone is different, and it’s also normal to have few symptoms. The most common include:

Tiredness, especially in the first 12 weeks
Nausea and vomiting, at any time of day
Sore breasts
Needing to pee more often
A metallic taste
Bloating, mild cramps and light spotting

What to do and what to avoid

Contact a midwife or doctor to start your care. Take 400 micrograms of folic acid a day until at least week 12, and vitamin D is also advised. The NHS says not to drink alcohol and to stop smoking. Don’t stop a prescribed medicine on your own, because that can be harmful too. Ask first.

Appointments and scans

In the UK, your first full appointment is usually at 8 to 12 weeks, and the 12-week scan is at about 10 to 14 weeks. You’ll also be offered optional screening. The US often has a first visit at around 8 to 10 weeks.

Miscarriage in the first trimester

Most miscarriages happen in this trimester, and the risk falls quickly as the weeks go on. Read what the studies show.

When to get help

Call your midwife, doctor or maternity unit straight away, day or night, if you have any of these:

Vaginal bleeding, with or without cramps.
Strong pain low on one side of your belly, pain at the tip of your shoulder, or feeling faint. These can be signs of an ectopic pregnancy, which needs urgent care.
Being sick so often that you can’t keep fluids down for a day.
Fever of 38 °C (100.4 °F) or higher, or pain or burning when you pee.
Pain or swelling in one leg, chest pain, or sudden shortness of breath. These can be signs of a blood clot.
Feeling very low, hopeless or anxious most days. If you feel you might harm yourself, get emergency help.

Based on NHS and ACOG guidance. This list isn’t complete. If something feels wrong, call anyway. In an emergency, call your local emergency number.