First Trimester Guide: What to Expect in Weeks 1 to 12

The first trimester covers weeks 1 to 12 of pregnancy. It’s a time of huge change: your baby grows from a tiny cluster of cells into a fully formed fetus about the size of a plum, and many people feel more tired and queasy than they ever expected. This guide walks you through what happens, what to do, and what to watch for.

This article is general information, not medical advice. Your midwife, doctor or OB-GYN is the best person to answer questions about your own pregnancy.

How pregnancy weeks are counted

Pregnancy is dated from the first day of your last period, not from the day you conceived. That means in “week 1” and “week 2” you aren’t actually pregnant yet. By the time you miss a period, you’re already counted as about 4 weeks pregnant.

If you haven’t taken a test yet, see our guide to early pregnancy signs and when to test.

Your baby’s development, week by week

Weeks 1 to 4

Ovulation, fertilization and implantation all happen in these weeks. By week 4, the embryo is about 2 mm long, roughly the size of a poppy seed. Some home pregnancy tests can pick up pregnancy from around this time.

Weeks 5 to 8

Growth is fast. By week 8 your baby is about 16 mm long, roughly the size of a raspberry, and is now called a fetus. The head is starting to uncurl, the arms are longer than the legs, and the placenta is forming and attaching to the wall of the womb.

Weeks 9 to 12

By week 12 your baby is about 5.4 cm (just over 2 inches) from head to bottom, roughly the size of a plum. The placenta has taken over feeding the baby, the sex organs have formed, and the heartbeat can be seen on an ultrasound scan.

Common first trimester symptoms

Not everyone gets all of these, and some people feel almost nothing. Common symptoms include:

• Tiredness, often more than you’d expect
• Nausea and vomiting (“morning sickness”), which can happen at any time of day
• Sore or swollen breasts
• Needing to pee more often
• A metallic taste in your mouth
• Headaches, bloating and mild cramping
• Mood swings
• Heartburn and dizziness, especially toward the end of the trimester
• Light spotting around the time the embryo implants

Many of these ease as the trimester ends and hormone levels settle. If vomiting stops you keeping food or fluids down, contact your doctor or midwife.

Your first appointments and scans

In the UK

• Refer yourself to NHS maternity services as soon as you know you’re pregnant, or contact your GP or midwife.
• Booking appointment (8 to 12 weeks): your first full appointment with a midwife takes around an hour. They’ll ask about your health, previous pregnancies, mental health, lifestyle and home situation, check your blood pressure and urine, and offer blood tests. You’ll get your maternity notes to keep with you.
• Dating scan (around 10 to 14 weeks): this ultrasound confirms how far along you are, works out your due date, checks whether you’re expecting more than one baby and looks at early development. It usually takes about 20 minutes.
• Screening: if your scan is between 10 and 14 weeks, you can choose to have the combined test for Down’s syndrome, Edwards’ syndrome and Patau’s syndrome (a blood test plus a measurement taken on the scan). Screening is optional.

First-time parents in England are offered at least 10 antenatal appointments and 2 ultrasound scans during pregnancy.

In the US

• Call your OB-GYN, family doctor or midwife to book your first prenatal visit.
• At the first visit you’ll usually have a physical exam and blood tests (including blood type and Rh factor, anemia, some infections, and immunity to rubella and chickenpox), and your due date will be calculated.
• First trimester screening is usually offered at 11 to 14 weeks (a blood test plus a nuchal translucency ultrasound).
• Routine checkups are usually about once a month until week 28, then more often as your due date gets closer.

Vitamins and supplements

• Folic acid: take 400 micrograms (mcg) a day, ideally from before you conceive and at least until week 12. It lowers the chance of neural tube defects such as spina bifida. Some people need a higher dose, so ask your doctor if you have diabetes, take certain medicines, or had a previous pregnancy affected by a neural tube defect.
• Vitamin D: the NHS recommends 10 mcg a day, especially from October to March. Don’t take more than 100 mcg a day.
• Avoid vitamin A (retinol) supplements and cod liver oil, because too much vitamin A can harm your baby.

What to eat and what to avoid

Aim for a varied, balanced diet. The NHS advises avoiding or limiting:

• Alcohol: avoid it completely. The CDC says there is no known safe amount of alcohol and no safe time to drink during pregnancy. If you drank before you knew you were pregnant, stopping now still helps your baby.
• Caffeine: no more than 200 mg a day. That’s roughly 2 mugs of instant coffee (about 100 mg each) or 2 to 3 cups of tea (about 75 mg each). Remember cola, energy drinks and chocolate contain caffeine too.
• Unpasteurized milk and dairy, and some soft cheeses
• Raw or undercooked meat, liver and liver products, and game meats
• Raw or partly cooked eggs, unless they carry the British Lion stamp (UK). In the US, choose pasteurized eggs or cook eggs until firm.
• Raw fish and shellfish, and shark, swordfish and marlin
• Tuna: no more than 4 cans or 2 tuna steaks a week. Oily fish (like salmon or mackerel): no more than 2 portions a week.

If you feel unwell after eating something you should avoid, or have food poisoning symptoms for more than 48 hours, get medical advice.

Stop smoking and vaping. Ask your doctor or midwife for free support to quit.

Staying active

For most people, exercise is safe and good for you in pregnancy. The general advice is 150 minutes of moderate activity a week, plus muscle-strengthening activity twice a week. “Moderate” means you can still talk but not sing.

• If you weren’t active before, start gently with about 10 minutes a day and build up.
• Don’t start a new strenuous sport, like running, for the first time now.
• Avoid contact sports and activities with a risk of falling or being hit in the tummy, and don’t scuba dive.

Looking after your mental health

Feeling anxious, low or overwhelmed in early pregnancy is common, especially when you’re tired and sick. Your midwife or doctor will ask about your mental health. Tell them honestly how you feel. Support is available, and asking for it is a strong step.

When to get help right away

Contact your doctor, midwife or early pregnancy unit (in the UK, NHS 111 if you can’t reach them) if you have:

• spotting or light bleeding, especially with mild pain
• unusual vaginal discharge or fluid leaking
• vomiting so much you can’t keep food or fluids down
• pain or burning when you pee

Call 911 (US) or 999 (UK), or go to the emergency room, if you have:

• bleeding with severe tummy pain
• heavy bleeding (soaking a pad)
• pain in the tip of your shoulder
• feeling very dizzy or faint, or passing out

These can be signs of a miscarriage or an ectopic pregnancy, which needs urgent treatment. Bleeding in early pregnancy doesn’t always mean something is wrong, but it should always be checked.

Sources

• NHS: Week-by-week guide: weeks 1 to 12
• NHS Best Start in Life: Week 4, Week 8, Week 12
• NHS: Your antenatal appointments
• NHS: 12-week scan
• NHS: Pregnancy vitamins and supplements
• NHS: Foods to avoid in pregnancy
• NHS: Exercise in pregnancy
• NHS: Miscarriage
• CDC: About Alcohol Use During Pregnancy
• Office on Women’s Health (HHS): Prenatal care and tests

Last updated: November 13, 2026