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4 Weeks Pregnant

At 4 weeks, your period is due and a home pregnancy test may now be positive. The embryo is about the size of a poppy seed.
Baby is about the size ofA poppy seed
LengthAbout 1 mm
WeightLess than 1 g
Week 4 of 40, first trimester36 weeks to go

Your baby at 4 weeks

The embryo is now settled in the lining of your uterus. Its cells are forming three layers that will become all of the baby's organs and tissues. The placenta and amniotic sac are also starting to form. Your body is making the pregnancy hormone hCG, which home tests detect.

Your body at 4 weeks

Your period is due around now. A missed period is often the first sign of pregnancy. You may also have mild cramps, tender breasts or feel more tired than usual.

Your checklist this week

  • Take a home pregnancy test from the first day of your missed period.
  • If it is positive, contact your doctor or midwife to arrange your first appointment.
  • Work out your due date with our Due Date Calculator.
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Not sure which week you are in?

Use our Due Date Calculator or Pregnancy Week Calculator to find out exactly how far along you are.

Your baby this week

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When to call your doctor

  • Bleeding from your vagina
  • Severe pain in your tummy
  • A bad headache, blurred vision or sudden swelling
  • Fluid leaking from your vagina
  • Fever, or pain when you pass urine
  • Later in pregnancy, your baby moving less than usual

In an emergency, call your local emergency number.

Based on ACOG and NHS guidance. Sizes are averages: every baby grows at its own pace.