4 Weeks Pregnant
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026
Quick answer: At 4 weeks pregnant, your baby is about the size of a poppy seed, about 1 mm (0.04 in) long. You're in the first trimester, in month 1, with 36 weeks to go until your due date. This week's milestone: positive test possible. Symptoms vary from person to person.
| Week 4 at a glance | |
|---|---|
| Trimester | First |
| Month | 1 |
| Weeks to due date | 36 |
| Baby's size | About the size of a poppy seed |
| Length | About 1 mm (0.04 in) |
| This week | Positive test possible |
Sizes are typical averages. Before about 20 weeks, length is measured crown to rump; after that, head to heel.
Your baby at 4 weeks
Your baby is now an embryo about the size of a poppy seed. The cells are sorting into layers that will become every organ, and the placenta and amniotic sac are starting to form.
Your body at 4 weeks pregnant
Your period is due around now, so this is often the week of a positive pregnancy test. Early symptoms such as tiredness, sore breasts or needing to pee more often can start between 4 and 6 weeks, according to the NHS.
Things to do this week
- Take a home pregnancy test if your period is late.
- Contact your doctor, midwife or local maternity service to start prenatal care.
- Check any medicines or supplements you take with a pharmacist or doctor.
When to call your doctor or midwife
Contact your care team straight away if:
- you have vaginal bleeding, especially with cramps or belly pain
- you have severe or one-sided belly pain, or pain in the tip of your shoulder
- you are vomiting so often you can't keep fluids down (the CDC flags being unable to drink for more than 8 hours)
- you have a fever of 100.4°F (38°C) or higher
- it hurts or burns when you pee
If something feels wrong, trust that feeling and call. In an emergency, call your local emergency number.
Track week 4 in Momi
Add your due date in Momi and start a pregnancy diary. Many parents like to write down how they found out. Momi follows your pregnancy week by week with your baby's size and development, and keeps your diary, bump photos, appointments, kick counts and contractions in one place.
Download Momi free on the App Store or Google Play. Free with ads; optional Premium.
Keep reading
← 3 weeks pregnant · All weeks · 5 weeks pregnant →
Sources
- ACOG: Changes During Pregnancy (infographic)
- NHS: Signs and symptoms of pregnancy
- ACOG: Morning Sickness: Nausea and Vomiting of Pregnancy
- CDC: Urgent Maternal Warning Signs
Medical disclaimer: Momi and this website provide general information, not medical advice. Always talk to your doctor or midwife; in an emergency call your local emergency number.
FAQ
How big is a baby at 4 weeks pregnant?
On average, a baby at 4 weeks is about the size of a poppy seed: about 1 mm (0.04 in). These are typical averages, and healthy babies vary.
How many months is 4 weeks pregnant?
4 weeks pregnant is in the first month of pregnancy and the first trimester. Months don't divide evenly into 40 weeks, so different sources may label the month slightly differently.
What should I do at 4 weeks pregnant?
Take a home pregnancy test if your period is late. Contact your doctor, midwife or local maternity service to start prenatal care.
Related guides
Track this in Momi
Momi is a free pregnancy tracker app for iPhone and Android that follows pregnancy week by week, estimates the due date, and keeps a pregnancy journal, weight log, kick counter and contraction timer in one place.
