Pregnancy week by week: a complete UK guide

Forty weeks sounds like a long time and a blur all at once. This is the whole journey laid out gently: how pregnancy is counted, what happens to you and your baby through each trimester, the NHS appointments to expect, how to look after yourself, and the signs that always deserve a phone call.

How pregnancy is counted

Pregnancy is measured as 40 weeks from the first day of your last period, not from conception. That sounds odd, because you were not pregnant during the first couple of those weeks, but it gives everyone a consistent starting line, since most people know when their last period began. It does mean you are counted as "two weeks pregnant" around the time you actually conceive.

Forty weeks is an average, not a deadline. Any birth from 37 to 42 weeks is considered a normal length, and only about 1 in 20 babies arrive on the exact due date. It really is a "due fortnight". If you want the maths, our due date calculator works it out from your last period, a conception date or an IVF transfer, and our guide to how due dates are calculated explains why the date can shift after your dating scan.

The first trimester: weeks 1 to 12

So much happens in these early weeks, even though there is nothing to see on the outside. From a tiny cluster of cells your baby grows a beating heart, the beginnings of a brain and spinal cord, and little limb buds. By week 12 they are recognisably a tiny person, around the size of a plum.

For you, this stage often brings tiredness, tender breasts and nausea that does not always keep to mornings. Some people feel completely well and some feel rotten, and both are normal. The things worth doing now: start taking folic acid if you have not already, contact your GP or midwife to book in, and look ahead to your dating scan. If you are still in the trying stage, our guide on how to get pregnant covers the run-up.

The second trimester: weeks 13 to 27

Often called the golden trimester, this is when many people start to feel like themselves again. The early nausea usually eases, energy returns, and a bump begins to show. Somewhere between 16 and 24 weeks you may feel those first fluttery movements, often later if it is your first baby.

Your baby grows quickly now, developing hearing, sleep and wake cycles, and a protective coating called vernix. The big appointment is the anomaly scan at around 20 weeks, a detailed check of your baby's growth and development, where you may be able to find out the sex if you want to. This calmer stretch is the ideal time to research bigger purchases, sort maternity leave, and start thinking about your birth preferences. Our trimester-by-trimester guide goes a little deeper on each stage.

The third trimester: weeks 28 to birth

The final stretch. Your baby puts on weight, their lungs and brain keep maturing, and they usually settle into a head-down position ready for birth. Your antenatal checks become more frequent, and you will talk through your birth plan, feeding and what to expect in labour.

Physically this trimester can be demanding, with backache, broken sleep, heartburn and a strong urge to nest all common. The practical jobs come into focus: fitting the car seat, packing your hospital bag, and getting first-size essentials ready. From 37 weeks your baby is considered full term, so it is worth being ready a couple of weeks before your due date rather than after it.

Your UK antenatal appointments

Your exact schedule depends on whether it is your first baby and your individual circumstances, and your midwife will tailor it, but the typical NHS pathway looks something like this:

• Booking appointment (by around 8 to 10 weeks). Your first, longest appointment, covering your health, history and the tests on offer.
• Dating scan (around 8 to 14 weeks). Confirms how many weeks pregnant you are and your due date.
• Anomaly scan (around 18 to 21 weeks). A detailed check of your baby's development.
• Regular checks through the second and third trimesters, measuring your bump, blood pressure and, when needed, urine, with more frequent visits as you near the end.
• Vaccinations. The whooping cough and flu vaccines are offered in pregnancy to protect you and your baby. Your midwife will tell you when.

Looking after yourself

• Take your supplements. 400 micrograms of folic acid daily until 12 weeks, and 10 micrograms of vitamin D daily throughout pregnancy, are the standard NHS recommendations. Some people need a higher folic acid dose, so check with your midwife.
• Eat well, and know the foods to avoid. A varied diet is the goal, while steering clear of a short list of foods such as unpasteurised cheeses, undercooked meat and high-mercury fish. The NHS has a clear list.
• Skip alcohol and smoking. The safest amount of each in pregnancy is none, and stopping helps at any stage.
• Keep moving. Staying gently active is good for you and your baby, no need for anything intense.
• Mind your mental health. Pregnancy stirs up a lot. If you feel low, anxious or simply not yourself, tell your midwife. It is common, and support is there.

Warning signs to never ignore

Most pregnancies are healthy, but some symptoms always deserve a prompt call to your midwife or maternity unit, day or night. Do not wait or worry about being a nuisance:

• Any vaginal bleeding.
• A severe or lasting headache, problems with your vision, or sudden swelling of your face, hands or feet, which can be signs of pre-eclampsia.
• A high temperature, or severe or persistent tummy pain.
• Severe or constant vomiting that stops you keeping fluids down.
• Any reduction or change in your baby's movements once they are established. This is important at any time of day, so call straight away.

Trust your instincts. If something feels wrong, you are always right to ring and ask.

Getting ready for baby, and beyond

You do not need to buy everything at once. The essentials for the early days are fairly short: somewhere safe for the baby to sleep, a car seat, feeding basics, nappies and a few first-size clothes. Our baby shopping timeline spreads it across the trimesters so nothing lands in a last-minute rush, and our hospital bag checklist covers the final pack for labour. Once your baby arrives, our early years calculator helps you keep track of their age and the milestones ahead.

Frequently asked questions

How many weeks is full term?

From 37 weeks your baby is considered full term, and a normal pregnancy can last up to 42 weeks. Before 37 weeks is premature, and your team will keep a closer eye if labour starts early.

When will I feel the baby move?

Usually between 16 and 24 weeks, often later with a first baby. Once movements settle into a pattern, get to know it, and call your midwife straight away if they slow or change.

How accurate is my due date?

It is an estimate, refined by your dating scan. Only around 1 in 20 babies arrive on the day, so treat it as the centre of a two-week window. Our guide to due dates explains why.

What vitamins should I take?

The standard NHS advice is 400 micrograms of folic acid until 12 weeks and 10 micrograms of vitamin D throughout. Avoid supplements containing vitamin A. Always confirm with your midwife, as your needs may differ.

The bottom line

Pregnancy is counted as 40 weeks from your last period, unfolds across three trimesters, and comes with a steady rhythm of NHS appointments to support you. Take your folic acid and vitamin D, look after the everyday basics, get ready gradually, and never hesitate to call your midwife about anything that worries you. Your due date is a guide, not a finish line.

Sources

• NHS, Pregnancy, including week-by-week, your antenatal care and keeping well.
• NICE, Antenatal care (NG201).
• Tommy's, pregnancy information and support.

This guide is general information, not medical advice, and cannot account for your individual circumstances. Always follow the guidance of your own midwife or GP. In an emergency, call 999, or contact your maternity unit or NHS 111 for urgent concerns.