4 weeks pregnant: development, early signs and next steps
4 weeks pregnant is a stage at which, the NHS says, you probably do not look pregnant yet, while an embryo about 2 mm long is growing inside (NHS, 2026). This page sets out what the NHS, ACOG and the Office on Women’s Health say about week 4, and where they count or measure differently.
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Medically reviewed by Dr Mary Davidson, Reproductive endocrinologist, checked on
Seek help now if you have any of these symptoms
Bleeding and cramping are common in early pregnancy and often do not mean miscarriage. The symptoms below, however, need to be assessed straight away.
- Soaking two or more sanitary pads per hour, or passing large clots
- Severe lower abdominal pain, especially on one side
- Pain at the tip of the shoulder
- Dizziness, fainting, or collapse
- Signs of shock — pale, clammy, rapid heartbeat, breathlessness
- Pain on opening bowels, or bladder or bowel changes
- Feeling very unwell, or feverish
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How the sources describe development around 4 weeks
- Size and shape, NHS
- The NHS week 4 page in its week-by-week guide (NHS, 2026) describes the embryo as about 2 mm long, about the size of a poppy seed, protected by an amniotic sac filled with cushioning fluid and attached to a yolk sac that provides its nourishment. It says the outer layer will later develop into the placenta.
- Development, ACOG
- ACOG groups weeks 1 to 8 together and does not break them down week by week. Its list for that stretch includes the brain and spine beginning to form, cardiac tissue starting to develop, the muscles of the eyes, nose and mouth forming, and the inner ear beginning to develop (ACOG, How Your Fetus Grows During Pregnancy, 2024).
- Size and development, Office on Women’s Health
- The Office on Women’s Health page on the stages of pregnancy (2025) says that at four to five weeks the brain and spinal cord have begun to form, the heart begins to form, arm and leg buds appear, and the embryo is one-twenty-fifth of an inch long. Each body gives its own measurement, and this page does not convert one into the other.
Weeks are counted from the first day of the last menstrual period, so about two weeks of the count come before conception (ACOG, How Your Fetus Grows During Pregnancy, 2024). This page covers 4 weeks 0 days to 4 weeks 6 days.
A normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, so at 4 weeks about 36 weeks remain to the estimated due date (ACOG, How Your Fetus Grows During Pregnancy, 2024). Your own due date depends on your last period or a scan, not on the week alone.
Your body at 4 weeks, as the NHS describes it
The NHS says you probably do not look pregnant yet, and that in a first pregnancy showing may not start until at least week 12. It adds that if this is not your first pregnancy you may start showing sooner, because the muscles of the uterus and belly may have been stretched before (NHS, 2026).
The NHS says that to start with you may have no symptoms at all. It lists changes that some people have in the first trimester, including a missed period, a metallic taste in the mouth, sore breasts, nausea, tiredness, needing to pee more often, light spotting and cramping like period pains, and it says that nobody else will have a pregnancy just like yours (NHS, 2026). On the same page it says to see your doctor if you get bleeding during pregnancy. Early pregnancy symptoms sets out the NHS list of early signs in full, and cramping in early pregnancy covers when cramps need checking.
Home tests and how the weeks are counted
The NHS says pregnancy is dated from the first day of your last period, and that some home tests can tell you that you are pregnant after about 3 and a half weeks and are 99% accurate (NHS, 2026). ACOG uses the same starting point: a normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, and about two of those weeks are counted before the pregnancy has begun (ACOG, 2024).
Booking your first appointment
In England the NHS says you can tell your GP or refer yourself to your local hospital, that the booking appointment usually takes place between weeks 8 and 12 and takes around an hour, and that the first dating scan is offered at 8 to 14 weeks (NHS, 2026). NICE guideline NG201 says the first antenatal appointment with a midwife should take place by 10 weeks 0 days (NICE, 2021). In the US, ACOG says it is best to start prenatal care in the first trimester and that before 10 weeks after your last period is ideal (ACOG, Prenatal Care, 2025).
Work out which week you are in
The Pregnancy week calculator takes your last period, conception date, scan or IVF transfer and shows how many weeks and days pregnant you are today.
Sources
- NHS Best Start in Life. Week 4 of pregnancy (week-by-week guide to pregnancy). nhs.uk, 2026.
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy (FAQ156). ACOG, 2024.
- Office on Women's Health, US Department of Health and Human Services. Stages of pregnancy. womenshealth.gov, 2025.
- National Institute for Health and Care Excellence. Antenatal care (NG201). NICE, 2021.
- American College of Obstetricians and Gynecologists. Prenatal Care (FAQ199). ACOG, 2025.
Frequently asked questions
How big is the baby at 4 weeks pregnant?
The NHS says the embryo is about 2 mm long, about the size of a poppy seed (NHS, 2026). The Office on Women’s Health gives one-twenty-fifth of an inch for weeks four to five (Office on Women’s Health, 2025). Each body gives its own measurement.
Can a home pregnancy test show a result at 4 weeks?
The NHS says some home tests can tell you that you are pregnant after about 3 and a half weeks, and describes them as 99% accurate (NHS, 2026). A midwife or doctor can answer questions about your own result.