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First trimester

3 weeks pregnant

At 3 weeks, conception may have happened: a sperm and egg join in a fallopian tube. Over the following days the fertilized egg divides into a ball of cells called a blastocyst, travels to the womb and begins to settle into its lining. It's usually too early for symptoms or a positive test.

This page covers 3 weeks 0 days to 3 weeks 6 days (3+0 to 3+6), counted from the first day of the last period. Check how far along you are

Enter it to see the calendar dates of your week 3.

Pregnancy weeks start before conception, so there is no embryo to measure yet.

How your baby is developing

  • Fertilization usually takes place in a fallopian tube, when a single sperm enters the egg. The combined cell, called a zygote, carries all the genetic information needed, half from the egg and half from the sperm.1,2
  • Within 24 hours of fertilization, the zygote starts dividing rapidly into many cells as it travels down the fallopian tube toward the womb. This journey takes about a week.1,2
  • Along the way it becomes a blastocyst: an inner group of cells that will become the embryo, and an outer layer that will form the membranes and placenta that nourish and protect it.1,2
  • Once the blastocyst reaches the womb, it begins to implant in the thickened lining. Implantation continues into week 4, when the blastocyst attaches firmly and starts receiving nourishment from your blood supply.1,2
  • After implantation, your body starts making pregnancy hormones. These stop your next period. The pregnancy hormone hCG starts to be produced around 6 days after fertilization.2,3

Changes you may notice

Everyone’s experience is different. You may notice some of these, or none.

  • Most people don't notice anything yet. Early pregnancy symptoms usually start around 4 to 6 weeks, and some people have no symptoms at first.4
  • Around the time your period would be due, some people have implantation bleeding: spotting or a very light bleed. Any bleeding in pregnancy is worth checking with a healthcare professional.4,5
  • In the second half of the menstrual cycle, the hormone progesterone helps prepare the womb lining for an embryo to implant.6

Care around this week

  • If you might be pregnant, keep taking 400 micrograms of folic acid a day. It's recommended for the first 12 weeks of pregnancy and reduces the chance of neural tube defects such as spina bifida.7,8
  • Not drinking alcohol, getting support to stop smoking and avoiding illegal drugs are recommended while trying to get pregnant. Don't stop any prescribed medicine without first talking to your doctor.9

Questions to ask your provider

  • When is the earliest a pregnancy test is likely to be accurate for me?
  • Is the medicine I take safe if I might be pregnant?
  • What should I do if I have spotting around the time my period is due?
When to contact your provider straight away

Call 911 (US) or 999 (UK) in an emergency, such as heavy bleeding or bleeding with severe pain, a seizure, chest pain, or severe trouble breathing.11,12,13,14

These symptoms are not always serious, but each one needs prompt advice from your provider, midwife or maternity unit (or NHS 111 in the UK if you cannot reach them) rather than waiting for your next appointment, and it helps to tell them you are pregnant.

  • Vaginal bleeding or spotting at any stage of pregnancy. Call 911 or 999 if the bleeding is heavy or comes with severe belly pain, shoulder pain, or feeling faint or dizzy.5,13
  • Fluid leaking or gushing from your vagina, or discharge that is unusual for you. Get checked straight away if the fluid is greenish, colored or smells bad.13,14,15,16
  • Your baby moving less than usual, not moving, or a change in their usual pattern of movements. Call immediately, even in the middle of the night, and do not use a home Doppler to check yourself.13,17
  • A severe headache that does not go away with simple painkillers or gets worse, or sudden changes in your vision such as blurring or seeing flashing lights.11,13,14
  • Sudden swelling of your face, hands or feet, or swelling, redness or pain in a leg.11,13,14
  • Belly pain that is severe or does not go away after resting, or pain just below the ribs.11,13,18
  • Regular tightenings or contractions, period-type pains, or backache that is unusual for you before 37 weeks, which can be signs of preterm labor.16,18
  • A fever of 100.4°F (38°C) or higher, or feeling hot, cold or shivery.13,14,16
  • Shortness of breath that comes on suddenly or happens when you lie flat, chest pain, a racing or pounding heart, or dizziness or fainting.13,14
  • Very itchy skin, especially on the palms of your hands or soles of your feet, or itching that is worse at night.19
  • Being sick so often that you cannot keep food or drink down, or signs of dehydration such as dark urine, peeing very little, or feeling dizzy.13,20
  • Thoughts of harming yourself or your baby. Get help right away, and call 911 or 999 if you think you might act on them.13,21

Questions about 3 weeks

Can a pregnancy test be positive at 3 weeks?

It's usually too early. Tests detect hCG, which is only starting to be produced around now. Most home tests are reliable from the first day of a missed period, although some very sensitive tests can be used a little earlier.3,10

What is a blastocyst?

It's the ball of cells the fertilized egg forms as it travels down the fallopian tube. Its inner cells will become the embryo, and its outer cells will form the membranes and placenta.1,2

Is spotting at 3 weeks implantation bleeding?

It can be. Light spotting around the time your period is due is sometimes implantation bleeding. Because bleeding in pregnancy has several possible causes, it's important to get advice from a healthcare professional.4,5

Sources

  1. Fetal developmentUS National Library of Medicine
  2. Fetal development: week-by-week stages of pregnancyCleveland Clinic
  3. Doing a pregnancy testNHS (England)
  4. Signs and symptoms that might mean you're pregnantNHS (England)
  5. Vaginal bleeding in pregnancyNHS (England)
  6. Periods and fertility in the menstrual cycleNHS (England)
  7. Pregnancy vitamins and supplementsNHS (England)
  8. About folic acidUS Centers for Disease Control and Prevention
  9. Trying to get pregnantNHS (England)
  10. Pregnancy testUS National Library of Medicine
  11. Pre-eclampsiaNHS (England)
  12. Epilepsy and pregnancyNHS (England)
  13. Urgent Maternal Warning Signs and Symptoms (Hear Her campaign)US Centers for Disease Control and Prevention
  14. Pregnancy symptoms you need to get help forNHS (England)
  15. Signs of going into labourNHS Best Start in Life
  16. Premature labour and birthNHS (England)
  17. Your baby's movementsNHS (England)
  18. Stomach pain in pregnancyNHS (England)
  19. Itching in pregnancyNHS (England)
  20. Severe vomiting in pregnancyNHS (England)
  21. Depression in pregnancyNHS (England)

Links checked November 12, 2026. Citing a source does not mean it endorses this site.

Written from the sources above and checked against them on 2026-11-12. Not reviewed by a clinician. Our editorial policy