🤰 First trimester
4 weeks pregnant
Your baby is about the size of a poppy seed
By Emma Whitfield · Pregnancy & Baby Writer
Your baby at 4 weeks
The fertilised egg has implanted in the womb lining and is now an embryo. The cells are organising into the layers that will become your baby's nervous system, organs and skin — and the placenta is beginning to form.
Your body at 4 weeks
This is often the week a missed period prompts a positive test. Many people feel nothing yet; others notice early tender breasts, mild cramping or extra tiredness. Implantation spotting can happen around now.
Common symptoms this week
- Missed period
- Mild cramping or implantation spotting
- Tender, heavier breasts
- Tiredness
Symptom lists are typical, never exhaustive — and having none of them is normal too.
Focus this week
Start (or continue) a 400mcg folic acid supplement and 10mcg vitamin D daily, and book in with your GP or self-refer to your local midwife service.
Week 4 checklist
- Take a pregnancy test from the first day of your missed period
- Start 400mcg folic acid and 10mcg vitamin D daily
- Book your first midwife (booking) appointment — self-referral works in most areas
4 weeks pregnant: common questions
Can I take a pregnancy test at 4 weeks?
Yes — 4 weeks pregnant corresponds to the first day of a missed period, which is exactly when home tests are designed to be reliable. Use first-morning urine, and retest in three days if it's negative but your period still hasn't arrived.
How big is my baby at 4 weeks pregnant?
At 4 weeks, your baby is approximately the size of a poppy seed. These are approximate averages — every baby grows at its own pace.
What's happening at 4 weeks pregnant?
The fertilised egg has implanted in the womb lining and is now an embryo. The cells are organising into the layers that will become your baby's nervous system, organs and skin — and the placenta is beginning to form. For you: this is often the week a missed period prompts a positive test. many people feel nothing yet; others notice early tender breasts, mild cramping or extra tiredness. implantation spotting can happen around now.
Which trimester is 4 weeks?
4 weeks pregnant is in the first trimester. The first trimester runs to the end of week 12, the second from weeks 13 to 26, and the third from week 27 to birth.
Sources & further reading
This is general information, not medical advice — every pregnancy is different. For anything that worries you, and for all your antenatal care, speak to your midwife or GP. If your baby's movements ever slow down or change, contact your maternity unit immediately, day or night. Reviewed July 2026.