4 weeks pregnant: baby's development, common symptoms and things to do

Here’s everything you need to know about week 4 of pregnancy, including common symptoms of early pregnancy and getting your antenatal care organised.

Week 4 of pregnancy

Congratulations on your exciting news!

A pregnancy is measured from the first day of your last period not the day you conceived your baby. This is because it's not always easy to be sure of the exact date you became pregnant.

For example, a fertilised egg may have implanted in your womb just 2 weeks ago, but if the first day of your last period was 4 weeks ago, this means you’re officially four weeks pregnant!

Pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period.

Use our due date calculator to find out when your baby is due. 

“I spent the entire week my period was due running to the loo, just to double check I didn’t have my period. My boobs felt tender - but that can be a sign of PMT too. I took an early test in the end and couldn’t believe it when it showed positive.”
Ayida, mum of one

Although a fertilised egg may have implanted in your womb just two weeks ago, if the first day of your last period was four weeks ago, you are officially four weeks pregnant! This may seem odd if you think you can definitely date the pregnancy more recently.

Get weekly updates on your baby's development from our expert midwives straight to your inbox.

Your baby’s development this week

The tiny person inside you is the size of a poppy seed. They are a little dot, measuring about 2mm.

The baby may be tiny but big things are happening. The fertilised egg has snuggled into the side of your womb. It divides into layers of cells that will eventually become different parts of your baby’s body.

Your pregnancy symptoms in week 4

Early symptoms of pregnancy

If you have regular periods (a regular menstrual cycle) the earliest and most reliable sign of pregnancy is a missed period.

Bleeding or spotting

You might notice some very light bleeding or spotting. This is known as implantation bleeding. This can be caused by the embryo planting itself in the wall of your womb. This is common and often happens around the time your period would normally be due.

If you notice any bleeding at any stage of your pregnancy, it's important to get it checked out by your doctor or midwife. Bleeding in pregnancy isn’t always a sign of something serious but it’s best to get it checked.

You may also have some period-like cramping in these early weeks.

Sore boobs

Pregnancy hormones can make your boobs larger and feel tender, heavy and sore – a bit like how they might feel before your period.

Breast awareness 

Your breasts change a lot during and after pregnancy, so it’s important to check them regularly and be aware of any unusual changes. This is called ‘breast awareness’. Breast awareness is important because some breast changes might be a sign of breast cancer. 

In collaboration with Tommy's, CoppaFeel! has produced a new resource especially for women and pregnant people about natural breast changes during and after pregnancy, tips on how to check your breasts and what to do if you notice any changes. Find out more about your breasts during and after pregnancy.

Other early pregnancy symptoms

Read more about early pregnancy symptoms, such as tiredness, needing to wee more often and mood swings. 

Find out more tips about getting through the first trimester.

What to do in week 4

Tell your local maternity unit or GP you are pregnant

You may be able to refer yourself to your local maternity unit. Visit your local hospital’s website for more information.

Contact your GP if you can’t self-refer, are on prescribed medication or have any health conditions, such as diabetes.

Talk to your healthcare professional about your medication

It is very important to tell your GP, midwife or specialist about any medication you may be on. They can make sure you are on the safest medication for you and baby. Do not stop taking any medication without talking to your GP or specialist first.

Your first appointment with the midwife usually takes place when you are around 8 weeks pregnant. This is called the booking appointment. In the meantime, reading through a few dos and don'ts of pregnancy may help.

Try to stop smoking and drinking alcohol or taking drugs in pregnancy

Smoking (including secondhand smoke) can cause serious complications in pregnancy and birth. Stopping is, by far, one of the best things you can do for your baby. If you have a partner, it’s also beneficial if they stop smoking too.

It can be difficult to stop smoking, but it’s never too late and there is lots of support available to help you.

Drinking alcohol can also cause serious complications, especially during the first 3 months. There is no known safe level for drinking alcohol during pregnancy, so you are recommended to avoid it. Find out more about alcohol in pregnancy.

Using illegal or street drugs during pregnancy, including cannabis, ecstasy or cocaine, can have a serious effect on your unborn baby. It's best not to stop abruptly without getting medical advice. This is because there may be withdrawal problems or other side effects. Your GP or midwife can help you.

Try not to be too hard on yourself if you were doing any of these things before you realised you were pregnant. Talk to your GP or midwife if you have any concerns about your baby.


Sexually transmitted infections (STIs) such as chlamydia, gonorrhoea and genital herpes can affect your baby's health during pregnancy and after their birth.

Some STIs don’t have any symptoms, so you may not know if you have one and you won’t be offered tests for STIs as part of your routine antenatal care. If you think you or a partner might have an STI, it’s important to go for a check-up as soon as you can. Here’s where you can you find your local sexual health clinic:

Don’t worry, nobody will judge you. All you’re doing is protecting yourself and your baby.

Continue (or start) taking supplements

Eating a healthy, varied diet in pregnancy will help you get most of the vitamins and minerals you need. But it’s also very important to take vitamin D and folic acid as supplements during pregnancy.

Vitamin D helps your baby's bones, teeth, kidneys, heart and nervous system to develop. If you’re pregnant or breastfeeding, it’s recommended that you take 10 micrograms of vitamin D a day.

Folic acid can help prevent birth defects known as neural tube defects, including spina bifida. To prevent this, it’s important to take a 400 micrograms folic acid tablet every day as soon as you find out that you’re pregnant and until you're 12 weeks pregnant.

You will be advised to take a higher dose of folic acid (5 milligrams) if you have a higher chance of your pregnancy being affected by neural tube defects. For example, if you have diabetes, take anti-epilepsy medicine or had gestational diabetes in a previous pregnancy.

Find out more about important supplements in pregnancy.

If you have a pre-existing mental health problem

If you have a pre-existing mental health condition and take medication, tell your doctor and the person who prescribed the medication that you’re pregnant as soon as possible.

They will talk to you about whether your medication is safe in pregnancy or whether you should consider a different treatment. It is very important that you do not stop taking medication without talking to your GP or specialist first.

Find out more about your mental wellbeing in pregnancy.

Contact the Tommy’s midwives

If you have any questions or concerns about your pregnancy, you can call our pregnancy line on 0800 014 7800 (Monday to Friday, 9am to 5pm), or email us at [email protected].

We only have one line available and you may not always get through straight away. Please leave us a message with your contact details and we will call you back as soon as possible.

1. NHS. Due date calculator. https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/ (Page last reviewed: 29 April 2021 Next review due: 29 April 2024)

2. NHS. Signs and symptoms of pregnancy. https://www.nhs.uk/pregnancy/trying-for-a-baby/signs-and-symptoms-of-pregnancy/ (Page last reviewed: 8 October 2019 Next review due: 8 October 2022)

3. NHS. Vaginal bleeding. https://www.nhs.uk/pregnancy/related-conditions/common-symptoms/vaginal-bleeding/ (Page last reviewed: 10 March 2021 Next review due: 10 March 2024)

4. Regan, Lesley (2019) Your pregnancy week by week: what to expect from conception to birth. Penguin Random House, London

5. NHS. Stopping smoking. https://www.nhs.uk/pregnancy/keeping-well/stop-smoking/ (Page last reviewed: 7 November 2019 Next review due: 7 November 2022)

6. NHS. Drinking alcohol while pregnant. https://www.nhs.uk/pregnancy/keeping-well/drinking-alcohol-while-pregnant/ (Page last reviewed: 29 January 2020 Next review due: 29 January 2023)

7. NHS. Illegal drugs in pregnancy. https://www.nhs.uk/pregnancy/keeping-well/illegal-drugs/ (Page last reviewed: 31 January 2019 Next review due: 31 January 2022)

8. NHS. Infections that may affect your baby. https://www.nhs.uk/pregnancy/keeping-well/infections-that-may-affect-your-baby/ (Page last reviewed: 10 March 2021 Next review due: 10 March 2024)

9. NHS. Vitamins, supplements and nutrition in pregnancy. https://www.nhs.uk/pregnancy/keeping-well/vitamins-supplements-and-nutrition/ (Page last reviewed: 14 February 2020 Next review due: 14 February 2023)

10. Royal College of Obstetricians & Gynaecologists. Gestational diabetes. https://www.rcog.org.uk/globalassets/documents/patients/patient-information-leaflets/pregnancy/pi-gestational-diabetes.pdf

Review dates
Reviewed: 11 July 2022
Next review: 11 July 2025