The three trimesters of pregnancy, week by week

Trimester is one of those words everyone uses and nobody quite explains. Here is the whole pregnancy in three chunks: what is happening to your baby, what you might feel and which appointments land in each stage in Ireland. Each section links to our week-by-week pages if you want more detail.

What are the three trimesters of pregnancy?

You will see slightly different cut-offs on different websites. We use the HSE's:

Trimester	Weeks	Main appointments in Ireland
First	Weeks 1 to 12	GP visit, booking visit and dating scan
Second	Weeks 13 to 27	Anatomy scan, whooping cough vaccine, diabetes test for many
Third	Week 28 to birth	More frequent check-ups, anti-D if you are rhesus negative

How are pregnancy weeks counted?

Your weeks of pregnancy are dated from the first day of your last period, not from the day you conceived. That means weeks 1 and 2 happen before you are actually pregnant, and fertilisation usually happens around week 3.

Pregnancy normally lasts about 40 weeks from that date. Our due date calculator works it out for you, and the dating scan at your booking visit checks it.

What happens in the first trimester (weeks 1 to 12)?

The first trimester is a very important time for your baby's development, even though from the outside nothing seems to be happening.

Your baby

• Week 4: the fertilised egg attaches to the lining of the womb. See 4 weeks pregnant.
• Week 5: about 2mm long, the size of an apple seed. The heart is forming as a simple tube.
• Week 6: the heartbeat can sometimes be seen on a vaginal ultrasound scan. See 6 weeks pregnant.
• Week 8: the embryo is now called a fetus, roughly the length of a raspberry. See 8 weeks pregnant.
• Week 10: the heart is fully formed.
• Week 12: about 61mm long, around the size of a plum. All the organs, muscles, limbs and bones are in place, and your baby is moving, though you can't feel it yet. See 12 weeks pregnant.

You

A missed period is often the first sign. After that, the HSE lists tiredness, more vaginal discharge, needing to pee more often and changes to your breasts. Morning sickness affects about 8 in 10 pregnant women. It usually starts between 4 and 7 weeks and can happen at any time of day.

Feelings can be all over the place too: excitement, joy, nerves and fear, sometimes in the one afternoon. That is normal. The HSE's practical advice is to rest when you can, drink about 2 litres of fluid a day and eat small meals often.

Appointments

• See your GP to confirm the pregnancy and register for the free Maternity and Infant Care Scheme
• Take 400 micrograms of folic acid a day, at least until 12 weeks
• Your booking visit at the maternity unit or hospital, usually between 8 and 12 weeks, with blood tests and a dating scan
• The flu vaccine, which the HSE says is free for pregnant women and safe at any stage

Our guide to what to do when you just found out you're pregnant covers this stage step by step.

What happens in the second trimester (weeks 13 to 27)?

For many people this is the easier stretch. The HSE says the second trimester often brings renewed energy, and nausea and tiredness may ease.

Your baby

• Week 13: about the size of a peach. See 13 weeks pregnant.
• Week 14: about 85mm from head to bottom, the size of a lemon. The body is covered in fine hair called lanugo.
• Week 15: your baby can start to hear sounds, including your voice and your heartbeat.
• Week 16: about 110mm long, the size of an avocado. See 16 weeks pregnant.
• Week 17: around 150g and about 13cm long, with fingerprints forming.
• Week 20: usually the halfway point. A white greasy coating called vernix protects the skin. See 20 weeks pregnant.
• Week 24: your baby has a chance of survival if born now with lots of support. See 24 weeks pregnant.
• Week 27: around 1kg, and the end of the second trimester.

You

You should start to notice your baby moving at some point in the second trimester. It may feel like fluttering or a little poke. Around 18 to 19 weeks it can feel like a soft fluttering or rolling, but don't worry if you can't feel anything yet, especially in a first pregnancy. The HSE says morning sickness usually clears up by 16 to 20 weeks.

Other common changes include back and pelvic girdle pain, swollen legs and ankles, bleeding gums and constipation. The HSE recommends sleeping on your side from the second trimester on. Forgetfulness and vivid dreams are common too.

Appointments

• Whooping cough vaccine: recommended between weeks 16 and 36. It is free at your GP if they take part in the Maternity and Infant Care Scheme
• Fetal anatomy scan: at around 20 weeks. The HSE gives a window of 18 to 22 weeks
• Gestational diabetes test: usually between 24 and 28 weeks. Some hospitals offer it to everyone, others only to women with risk factors

This is also a good time to look into your maternity leave. Our guide to maternity leave and Maternity Benefit in Ireland explains the notice periods and payments.

What happens in the third trimester (week 28 to birth)?

The home stretch. Your baby is mostly growing and getting ready for life outside.

Your baby

• Week 28: about 370mm long, the size of an aubergine. See 28 weeks pregnant.
• Week 32: your baby is usually lying head down, ready for birth. See 32 weeks pregnant.
• Week 33: the brain and nervous system are fully developed.
• Week 36: about 48cm long, and the lungs are fully formed and ready for a first breath. See 36 weeks pregnant.
• Week 37: your pregnancy is considered full term. The average baby weighs around 3kg to 4kg by now. See 37 weeks pregnant.
• Week 40: about 510mm long. See 40 weeks pregnant.

You

You may notice Braxton Hicks contractions, tightenings that are usually irregular and usually last less than a minute. Heartburn is common as your baby presses on your stomach, and so are stretch marks, piles and varicose veins. The HSE says it is normal to feel more anxious about the birth and about becoming a parent.

Appointments

Your appointments with your GP, midwife or obstetrician become more frequent. From 28 weeks they check your baby's growth, development and position, and look out for problems like high blood pressure. If you are rhesus negative, you may be offered an anti-D injection at 28 weeks. You may not need another scan after the 20 week one unless you are advised to have one.

The most important thing in this trimester is your baby's movements. The HSE is clear that it is not true that babies move less towards the end of pregnancy. If your baby's movements change, slow down or stop, contact your midwife or maternity unit straight away.

It is also time to think about antenatal classes and to pack. Our hospital bag checklist for Ireland can help.

What if I go past 40 weeks?

A due date is an estimate. The HSE says labour usually starts within about a week either side of it, and you might go overdue. Your midwife or obstetrician will talk you through what happens next if you do. Our 41 weeks pregnant page covers that stage.

When should I call my maternity unit, whatever the trimester?

At any stage, contact your GP, midwife or maternity unit straight away if you have:

• Bleeding from the vagina or severe tummy pain
• A severe headache, especially with blurred vision or swelling
• Pain when peeing or a high temperature
• A change in your baby's usual pattern of movements

In an emergency, call 112 or 999.

If you'd like to keep track of which week you are in, the Lullagram app has a free pregnancy section that follows your baby week by week from your due date.

Frequently asked questions

What weeks are the first, second and third trimester?

Using the HSE's week boundaries, the first trimester runs to the end of week 12, the second trimester is weeks 13 to 27 and the third trimester runs from week 28 until your baby is born.

When does the second trimester start?

The second trimester starts at week 13 of pregnancy. For many people it brings more energy, and the HSE says morning sickness usually clears up by 16 to 20 weeks.

When will I feel my baby move?

You should start to notice movements at some point in the second trimester. They may feel like fluttering or a small poke at first. In a first pregnancy you may not feel them as early.

What scans do you get in pregnancy in Ireland?

You will usually be offered a dating scan at the booking visit, usually between 8 and 12 weeks, and a fetal anatomy scan at around 20 weeks. You may not need another scan after that unless your midwife or doctor advises one.

When is a baby full term?

At 37 weeks your pregnancy is considered full term. Pregnancy normally lasts about 40 weeks, and labour usually starts within about a week either side of your due date.