Week 4 of 40
What's happening with the baby
Pregnancy weeks are usually counted from the first day of your last period, not the day you conceived. So at four weeks, fertilisation was often about two weeks ago, and the cluster of cells has usually reached the uterus and started to implant. The NHS week 4 guide and Pregnancy, Birth and Baby both describe this early stage that way.
By the end of the week the embryo is still tiny (often compared to a poppy seed, around 2mm). Cells are sorting into layers that will become the nervous system, organs and other structures. Outer parts start forming what will become the placenta and amniotic sac. ACOG's overview of fetal growth is a clear plain-language walk-through.
You probably will not look pregnant yet. Many people in a first pregnancy do not show until later in the first trimester.
What you might notice
Some people feel nothing unusual yet. Others notice a missed period, sore breasts, mild cramping, tiredness, a stronger sense of smell, needing to pee more, or light spotting. Tommy's week 4 page covers these early signs in more detail.
Light spotting can be implantation bleeding: a small amount of pink or brown discharge as the embryo settles in. It is usually lighter than a period and short-lived. The NHS still advises getting any bleeding in pregnancy checked, because you cannot always tell what is going on from home.
Home tests look for the hormone hCG. Some sensitive tests can show positive from about three and a half weeks after your last period, but a test around a missed period is a common time to try. If it is negative and your period still does not arrive, test again in a few days or speak to a GP or clinic.
Things worth doing
- Folic acid. Many expecting parents take 400 micrograms (400 mcg) of folic acid daily until at least week 12 to help the neural tube form, but if you're unsure about the dose (or whether you need a higher one), check with your doctor or midwife. The NHS vitamins page explains the usual advice and when a higher dose is used.
- Book care. Contact a GP, midwife or maternity service to start antenatal care. In many places the longer first "booking" visit is around weeks 8 to 12. You do not have to wait if you have health conditions or take regular medicine. See the NHS antenatal care overview.
- Medicine check. Tell your doctor or specialist you are pregnant (or might be) before stopping or changing any prescription. Do not stop regular medicine on your own. The NHS medicines-in-pregnancy page is a good starting point for that conversation.
- Smoking and alcohol. If you smoke or drink, stopping now helps. There is no known safe amount of alcohol in pregnancy. Ask for support if quitting is hard.
- Vitamin D. Many expecting parents take a daily vitamin D supplement alongside folic acid, but if you're unsure whether you need it or which dose fits where you live, check with your doctor or midwife. Again, the NHS supplements guidance is useful.
For partners this week
Finding out (or waiting to find out) can stir up a mix of feelings for both of you. Tommy's guide for partners is written for this moment.
- Ask how they want to be supported: practical help, quiet company, or space.
- Offer to go with them to the GP or midwife, or to sit with them while they book.
- If you smoke, try to stop or keep smoke well away from them. Secondhand smoke still matters.
- Help track questions for the first appointment (medicines, symptoms, worries).
When to get help
Contact a doctor, midwife or urgent care service if you have:
- bleeding that worries you, bleeding with clots, or bleeding heavier than light spotting
- severe abdominal or shoulder-tip pain
- dizziness, fainting, or feeling very unwell
- pain when peeing, fever, or other signs of infection
Early bleeding is common and is not always a miscarriage, but it should be assessed. If you have had an ectopic pregnancy before, or you have sudden one-sided pain, seek care promptly. See the NHS bleeding page and ACOG's bleeding FAQ.