Your week-by-week guide to the first trimester of pregnancy

From the first signs of pregnancy to your 12-week scan, here’s what’s happening to you & your baby in the first trimester of pregnancy


The first trimester of pregnancy is the most important in a baby’s development – incredible things are happening to your body too. Find out about the exciting developments for you and your growing baby, with our week by week guide to the early months of pregnancy.

Week 1

Your pregnancy is calculated from the first day of your period so you’re not actually pregnant yet!

What’s happening to my baby?

There’s no baby, or indeed no embryo yet. At this stage your ‘baby’ is an immature egg inside one of your ovaries …waiting to become your bundle of joy!    

What’s happening to me?

Your pregnancy is calculated from the first day of your period, so that’s what’s happening to you right now! Write down the date, so if you fall pregnant, you’ll know the day your pregnancy ‘began’.

Week 2

What’s happening to my baby?

Er… still not much! Your eggs are being stimulated by hormones to mature, but only one will be released from an ovary usually around 10–16 days before your next expected period

What’s happening to me?

Dependent upon your own menstrual cycle, you may ovulate during week 2 –although some women will ovulate towards the beginning of week 3. Some women may recognize when ovulation happens as they may find their cervical mucus becomes clearer and thinner and others may feel other slight changes including bloating and mild tummy pain but these aren’t reliable ways of predicting ovulation. The best way to predict ovulation is to use an ‘ovulation test’ which measures hormones in your urine. 

After ovulation an egg lives for about 12-24 hours and for you to get pregnant a sperm must fertilize the egg within this time. Sperm can live for up to seven days inside your body. For the best chance of success, you should have sex every two to three days throughout the month not just when you ovulate. However, you are most likely to get pregnant if you have sex within a day or so of ovulation. 

Did you know that as soon as egg and sperm meet, your baby’s genetic makeup is complete?

Once your period has finished, the lining of your womb thickens again, and your body starts preparing to release another egg. It's recommended you take 400 mcg (micrograms) of folic acid daily while trying to conceive and during the first 12 weeks of pregnancy (to reduce the risk of neural tube defects) so start taking a suitable supplement as soon as you start trying to conceive. You should also try to follow a healthy diet and limit your alcohol intake if you are trying to get pregnant. During your pregnancy speak to your pharmacist before taking any over the counter medicines and talk to your GP if you’re on any prescription medication to check it’s suitable to take during pregnancy.

Week 3

What’s happening to my baby?

The newly fertilized egg begins as a single cell which begins to divide again and again as it sets off down the fallopian tube on its travels. By the time the egg reaches the thickened lining of your womb, it’s become a fluid-filled ball of over 100 cells. The cells on the surface will later become the placenta while the ones in the middle will develop into your baby. The embryo, as it’s now called, nestles itself into the lining of your uterus and gets comfy and forms links with the mother’s blood supply. The timing of this implantation depends on when ovulation and fertilization occurred, but it’s generally the end of week 3 to the beginning of week 4.

What’s happening to me?

Apart from bursting with impatience as you wait to find out if you’re pregnant or not, you’re probably not feeling much different to be honest! Hormonal changes in the first week or two following conception may cause some early symptoms such as painful, heavy breasts or fatigue; but many women feel relatively ‘normal’ and still don’t know. What you can do while you’re waiting is make sure you are having a healthy balanced diet which contains lots of calcium (cheese, milk, green veggies) and protein (lean meats, fish and eggs) for developing tissues, bones and teeth.

Week 4

What’s happening to my baby?

Your embryo is now a mass of cells up to 1mm long and has three layers – the inner layer will become the lungs, liver, digestive system and pancreas, the middle layer the skeleton, muscles, kidneys, blood vessels and heart; and the outer layer will form the nervous system, teeth and skin. In these early weeks, the embryo is attached to a tiny yolk sac that provides nourishment, but by week 12, the placenta will be fully formed and will take over the transfer of nutrients to the embryo.

What’s happening to me?

When the embryo implants, you might notice some light bleeding, (usually pinkish spotting) which is normal. That said, any bleeding in pregnancy should always be checked out with your doctor, it’s often not caused by anything serious but it’s important to make sure your period was probably due this week, but once the egg is implanted a hormone is released to tell your body to stop producing eggs and therefore your periods stop. Clever stuff. You may start feeling a little nauseous, tired and irritable – it can feel a lot like PMT, but these are also early pregnancy signs. You’re probably desperate to take a pregnancy test but try to wait until your period is at least a day late. 

Week 5

What’s happening to my baby?

Your baby is now 2-4mm in length – about the size of an apple pip!  This week’s development is focused on the head as the brain needs to undergo some serious growth, so it can look after the rest of your baby’s vital functions such as heart rate and blood circulation. Your baby’s heart is now made up of two tiny tubes (which are both already beating – ahhh!).

What’s happening to me?

If you resisted taking an early pregnancy test, this may well be the week to take a pregnancy test, when you discover that your life is about to change forever. Best start enjoying lie-ins because you won’t get them once baby arrives!

Your pregnancy symptoms might start to ramp up too – you may feel nauseous; your nipples may darken; you may get constipation; you may develop a strange metallic taste in your mouth; you may need to wee more, and you may go off certain foods or smells. You may get ALL these symptoms; you may get none… that’s just the way life goes.

After a positive pregnancy test, you’ll need to see your GP who’ll book you in with a midwife. It’s advisable to take 10mcg of vitamin D daily during pregnancy, especially during autumn and winter, although some women find it easier to take a special pregnancy supplement which contains everything you might need.

Week 6

What’s happening to my baby?

Your baby’s 6-8mm long, curving into a C-shape and quite frankly looks like a lentil-sized tadpole! Thickened circles will form on both sides of the head which will later develop into the eyes – and the passageways of the inner ears will begin to develop. The liver cells are now beginning to form before the rest of the digestive system, and small ‘limb buds will appear where the baby’s arms and legs will grow. The first thin layer of transparent skin appears covering the embryo and their little heart is now beating 90-110 times a minute.

What’s happening to me?

Fluctuating hormones mean you’re probably a little tetchy, and don’t be surprised if you’re nodding off on the sofa after dinner. It’s tiring work growing a human, you know! Even if you’re shattered, it’s good to keep up with gentle exercise if you can – it will give you more energy and keeping fit means you’ll cope better with the demands of pregnancy and birth. Coffee lovers will need to curb it a little in pregnancy as too much caffeine (also found in tea, cola and chocolate, remember) can increase your risk of miscarriage. It's recommended no more than two mugs of instant coffee a day. 

Week 7

What’s happening to my baby?

By now, your little one is about 10mm from head to bottom. Their facial features will become more detailed this week while the brain continues to grow rapidly, giving that huge forehead look you’ll see in pictures. Week 7 sees the esophagus and lungs rocking up while limb buds grow longer and begin to resemble limbs. Although not yet fully formed, the placenta is now up and running, giving your baby oxygen and nutrition and getting rid of waste. All in all, a busy week!

What’s happening to me?

You won’t know it’s happening, but a mucus plug will seal the entrance to your womb to protect your baby from infections. As the baby grows, so does your womb, which will be about the size of a lemon by now. Your appetite has probably increased although you may be put off foods you used to love!

Remembercertain foods should be avoided in pregnancy as they may make you ill or harm your unborn baby these can include:

• Soft, unpasteurized cheese like goats, feta, Brie, Camembert, and Stilton and other blue cheeses

• Unpasteurized milk and juices

• Raw or undercooked meats, including pâté, and fish and shellfish

• Fish that may contain high mercury levels such as shark, marlin and swordfish 

• Non-Lion-code raw or softly cooked eggs

Week 8

What’s happening to my baby?

Congratulations! Your embryo is now officially a fetus!  Your baby is beginning to uncurl; legs and arms are getting longer and forming into distinct sections. Facially, the eyes, nose, upper lip, outer ear and lower jaw will all become established. Meanwhile, the placenta is continuing to develop, forming structures that help attach it to the wall of the womb.

What’s happening to me?

By now, morning sickness may have well and truly kicked in – and not just in the morning either. If you are nauseous, get plenty of rest and eat little and often. It’s not nice, but morning sickness will normally pass, with most nausea disappearing by week 14. The volume of blood is increasing in your body, so you might also get thirsty. Try and drink at least eight glasses of water a day – even though that will mean lots of toilet trips.

If you own a cat, you need to be careful in your first trimester, as cat litter and faces contain a parasite that causes toxoplasmosis – a very rare infection that can cause miscarriage or stillbirth. You don’t have to give your cat away, just get someone else to change the litter tray.

Week 9

What’s happening to my baby?

Whoop! Baby’s had a growth spurt and is now as big as a grape (although you may be feeling decidedly larger). The features of the face that will be forever be etched into your heart are starting to develop further now, with eyes gaining some pigment, eyelids forming and even taste buds coming into the world. Brain, heart, lungs, kidneys and digestive tract are all there now although they still need some fine tuning. Primitive cells are also making their ways downwards to settle into male or female genitalia – mind blown!

What’s happening to me?

Your waist may be thickening, and your boobs are probably breaking previous records. Your fluctuating hormones may throw up periods of anxiety, which are perfectly natural. You should also have your ‘booking’ appointment in the diary by now – this will be with a GP or a midwife. You’ll discuss your general health and be offered a range of tests, including blood tests, to check that everything is OK. They’ll also book your first ‘dating’ ultrasound at around 12 weeks to check when baby is due and that all is developing normally. Write down any questions you may have to make the most of your appointment.

Week 10

What’s happening to baby?

By week 10, all your baby’s major organs are functional and starting to work together. The brain has connected itself to newly formed muscles and nerves and is now telling the body what to do – so your little one can start moving their limbs and even learn to swallow. Toes and fingers are well on their way as are testes and ovaries, the penis and the clitoris. And for those of you who like a visual interpretation, your baby is the size of a prune by now (but much prettier of course). 

What’s happening to me?

OK, so what might week 10 be throwing up for you? Or, it might be you that’s still throwing up. You may also be dealing with visible veins on your boobs (lots more blood being transported around); bleeding gums (so take care of teeth, remember, all routine dental work is free when you’re pregnant); an increase in clear or white vaginal discharge (all normal, but if it’s smelly, see a doc) and perhaps an increase in constipation (try to eat lots of high-fiber foods). 

Week 11

What’s happening to my baby?

Congratulations madam, you’ve got an apricot in there! From now until 20 weeks, your baby is going to do a lot of growing. Head and body are now more equal in size and an outer layer of skin is beginning to develop. The bones of the face have all, but formed and even hair follicles are appearing. This is the start of a super-active phase for your little one as he or she starts to move around in the amniotic fluid and test out those new limbs, muscles and joints! 

What’s happening to me?

Fingers crossed, the running to the toilet mid-way through a meeting has stopped by now. But even if you’re not vomiting anymore; you’re likely to be dog tired, as you’re involved in the last push to develop the placenta, which will be your baby’s life support for the rest of your pregnancy. Your pelvic floor muscles are constantly being stretched too, so you may find you leak a little while sneezing or laughing. Yes, we know they’re a pain, but pelvic floor exercises are so beneficial and now’s a great time to start them. Ask your midwife if you don’t know how!

Week 12

What’s happening to my baby?

The riskiest period of your pregnancy is now ending as all your baby’s critical systems are in place (brain, digestive system and heart); all organs are fully functional, and the placenta is fully developed, transferring oxygen and blood from you to your baby, who’s now about 6cm in size. The thrill of hearing that beating heart is part of your 12-week scan – as is seeing your baby squirm and kick on the screen. During the scan, you’ll be offered screening for Down's, Edwards' and Patau's syndromes as well as having your baby’s measurements and development checked and getting a closer estimation of the due date.

What’s happening to me?

You may be developing a tiny bump and your waist will be thickening a little. Don’t be surprised if you’ve already put on a few pounds. Hopefully by now, the tiredness should be lessening, and morning sickness is tailing off; although heartburn is quite common now due to hormonal changes that allow gastric acid to travel up the esophagus, causing a burning sensation. Eating little and often may help or speak to your pharmacist who can recommend a suitable product to help. Annoying symptoms aside, it’s a great feeling to make it to the end of the first trimester, knowing all is well. Give yourself a gentle pat on the back – or the bump!