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3 Weeks Pregnant

In week 3, the fertilised egg travels to your uterus and starts to implant in its lining.
Baby is about the size ofA pinhead
LengthAbout 0.1 mm
WeightLess than 1 g
Week 3 of 40, first trimester37 weeks to go

Your baby at 3 weeks

If an egg was fertilised, it is now a tiny ball of cells called a blastocyst. It divides as it travels down the fallopian tube to your uterus, and around the end of this week it starts to attach to the uterus lining. This is called implantation.

Your body at 3 weeks

You probably will not notice anything yet. A few people have light spotting around the time of implantation, which is much lighter than a period.

Your checklist this week

  • Avoid alcohol, smoking and recreational drugs.
  • Keep taking folic acid.
  • It is too early for a home test: wait until the first day of your missed period. Our pregnancy test calculator shows the best day.
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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.