pregnancy week 4
4 weeks pregnant.
Your baby is the size of a poppy seed. The embryo is implanting and forming three cell layers that will become every system in the body.
Updated 29 July 2026 · Sources: NHS and ACOG
Your baby this week
A tiny disc of three layers. The outer layer (ectoderm) becomes skin and brain, the middle (mesoderm) becomes bones and muscles, and the inner (endoderm) becomes organs.
- Implantation completes
- Neural tube begins forming
- Heart starts as a tube
- Placenta and amniotic sac form
What may be changing in your body
You may notice your missed period this week. Early hormones can bring breast tenderness, mild cramping, fatigue, or light spotting from implantation. A home pregnancy test is now reliable.
Missed period
The classic first sign. If your cycle is regular and your period is late, a home test taken now is usually accurate.
Implantation spotting
A light pink or brown smear lasting a day or two can occur as the embryo embeds in the uterine lining. It is lighter and shorter than a period.
Mild cramping
Period-like cramps without a period are common as the uterus responds to implantation. Mild, intermittent cramping at this stage is normal.
Breast tenderness
Rapidly rising hormones can make breasts sore, heavy or tingly — often the first symptom people notice. A softer, supportive bra helps.
Fatigue
Progesterone climbs steeply and acts as a natural sedative. Feeling wiped out by mid-afternoon is one of the earliest and most universal signs.
Heavy bleeding with pain — when to call
Spotting is common, but bleeding as heavy as a period with cramping or one-sided pain should be checked. Call your midwife or doctor to rule out early loss or ectopic pregnancy.
Your week 4 checklist
- Take a home pregnancy test with first-morning urine
- Contact your GP or midwife service to register the pregnancy and get the booking process started
- Keep taking 400 mcg folic acid daily — the neural tube is forming right now
- List any regular medications and ask your doctor or pharmacist which are safe to continue
- Check if you are up to date with vaccinations your provider may ask about
A useful partner move this week
If the test is positive this week, your first job is simply to be present for the news — whatever the emotion, let it land together. Practically, you can register with the GP or midwife service on her behalf and quietly take over the riskier kitchen jobs like handling raw meat. Keep the news within whatever circle you both agree on.
What comes next
The heart starts beating next week — a major early milestone. Pregnancy symptoms may begin showing up soon.
Common questions
How accurate is a pregnancy test at 4 weeks?
Taken on or after the day of your missed period, a good home test is around 99 percent accurate when used correctly. First-morning urine gives the most concentrated hCG and the clearest result. A negative test with a late period is worth repeating after two to three days, since hCG doubles roughly every 48 hours.
Is implantation bleeding different from a period?
Yes, in three ways: it is lighter, shorter and earlier. Implantation spotting is usually pink or brown, lasts from a few hours to two days, and does not build into a proper flow. If bleeding becomes as heavy as your normal period, treat it as a period — or, if you have a positive test, call your midwife or doctor.
My test is positive but I feel completely normal — is that okay?
Completely. Symptoms at 4 weeks range from nothing at all to full-blown fatigue and nausea, and both ends of that spectrum are normal. Symptom intensity does not predict how the pregnancy will progress. Many people feel entirely themselves until week 6 or beyond.
How many months is 4 weeks pregnant?
Pregnancy months do not divide evenly into weeks, so clinicians date pregnancy in weeks. At 4 weeks, you are in trimester one.