Everything you need to know about the first trimester (weeks 1 to 12)

You’re pregnant: congratulations. The first weeks of your pregnancy are a vital time as your body gets busy building a baby. 

On this page

First trimester: key stages

Not sure whether you are pregnant?

When am I due?

When will I see a midwife?

Keeping your baby safe

Your physical and mental health in pregnancy

Track your baby's development

First trimester: key stages

The first trimester begins on the first day of your last menstrual period and lasts until the end of week 12. This means that by the time you know for sure you're pregnant, you might already be 5 or 6 weeks pregnant.

A lot happens during these first 3 months. The fertilised egg rapidly divides into layers of cells and implants in the wall of your womb where it carries on growing. These layers of cells become an embryo, which is what your baby is called at this stage.

During this trimester, your baby grows faster than at any other time. By week 5, all the building blocks for your baby’s vital organs are in place. By week 12, your baby is starting to look more like a tiny human. They even have their own unique fingerprints.

Not sure whether you are pregnant?

Find out about the symptoms that mean you may be pregnant.

When am I due?

Find out when you are due with our due date calculator.

When will I see a midwife?

Your first midwife appointment (also known as an antenatal appointment) is the 'booking' appointment. This usually happens between week 8 and 10 of your pregnancy.

Find out how to register with a midwife and when your appointments will be.

Find about more about what a midwife does.

Keeping your baby safe

There are some things that you can do during pregnancy that have an effect on your baby. Find out about the dos and don'ts for a safer pregnancy.

Your physical and mental health in pregnancy

It’s common to experience symptoms such as pregnancy sickness (also known as morning sickness), tiredness and sore breasts. The first 12 weeks of pregnancy can be tough, but we have some useful tips for coping with common pregnancy symptoms.

Exercise is good for you and your baby and it doesn’t have to be strenuous to make a difference. Read more about exercise and pregnancy

Don't forget that your mental health is just as important as your physical health. It's natural to feel some anxiety and stress, but you may need some help if you’re struggling with these feelings. 

You can talk to your GP or midwife about how you feel at any time. They will help you find ways to manage your symptoms. They may ask if you would like to be referred to a specialist in mental health if you are really struggling. Be honest about how you feel so you can get the right support.

Read more about mental wellbeing in pregnancy

Symptoms to look out for

There are some symptoms to look out for in pregnancy. Having any of these does not necessarily mean there is anything wrong. But it’s always best to get checked out, just in case.

Read about the symptoms to look out for in pregnancy.

If you are Black or Mixed-Black it is even more important to tell your midwife or doctor if there is anything that you're worried about. 

Do not worry if you've talked about it before and don't be concerned about whether you're wasting anyone's time. This is your pregnancy and it's important to trust your own instincts if you feel something isn't right.

If you feel that you are not being heard get a second opinion. Five X More is a Black maternal health organisation. They have put together 6 steps to help you speak up for yourself in pregnancy.

We run a Black and Black Mixed-Heritage helpline. If you have any concerns book a call and we will support you.

The Tommy's Midwives' Helpline is a free phone line for anyone who needs advice, reassurance or support on any pregnancy or planning for pregnancy issue. It’s open 9-5, Monday to Friday on 0800 0147 800. You can also email questions to [email protected]

Track your baby's development

Sign up to a free pregnancy email from our midwives to track your baby's development and give you reminders of all you need to know through the 9 months of pregnancy. Click here to sign up.

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Mind (2024) Perinatal anxiety. Available at https://www.mind.org.uk/information-support/types-of-mental-health-problems/postnatal-depression-and-perinatal-mental-health/perinatal-anxiety (Accessed March 2025) (Page last reviewed: April 2024. Next review due: April 2027)  

NHS (2022) Signs and symptoms of pregnancy. Available at https://www.nhs.uk/pregnancy/trying-for-a-baby/signs-and-symptoms-of-pregnancy/ (Accessed March 2025) (Page last reviewed: 29 December 2022. Next review due: 29 December 2025)  

NHS (2023) Your antenatal appointments. Available at https://www.nhs.uk/pregnancy/your-pregnancy-care/your-antenatal-appointments (Accessed March 2025) (Page last reviewed: 21 April 2023. Next review due: 21 April 2026)  

NHS (2023). Exercise in pregnancy. Available at https://www.nhs.uk/pregnancy/keeping-well/exercise/ (Accessed March 2025) (Page last reviewed: 15 March 2023. Next review due: 15 March 2026)  

NHS Inform (2024) How your baby develops week to week - The first 8 weeks. Available at https://www.nhsinform.scot/ready-steady-baby/pregnancy/your-baby-s-development/how-your-baby-develops-week-to-week/#the-first-8-weeks (Accessed March 2025) (Page last reviewed 4 March 2024) 

NHS Start for Life (nd) 4 Weeks Pregnant. Available at https://www.nhs.uk/start-for-life/pregnancy/week-by-week-guide-to-pregnancy/1st-trimester/week-4/ (Accessed March 2025)

NHS Start for Life (nd) 8 Weeks Pregnant. Available at https://www.nhs.uk/start-for-life/pregnancy/week-by-week-guide-to-pregnancy/1st-trimester/week-8/ (Accessed March 2025)
 

Review dates
Reviewed: 11 June 2025
Next review: 11 June 2028