you're 4 weeks pregnant.
last revised · baby is about 0.1 cm (0.04") · 0.04 g (0.00 oz)
At 4 weeks pregnant, your baby is about 0.1 cm and 0.04 g — Your baby is the size of a poppy seed.
about the size of a…
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.
this week's highlights
- Implantation completes
- Neural tube begins forming
- Heart starts as a tube
- Placenta and amniotic sac form
how your baby looks
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.
body in focus
- head
- Neural tube begins forming — it will become the brain and spinal cord.
- body
- Three germ layers form, each destined to build entire body systems.
- hands
- feet
what's happening 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.
symptoms at 4 weeks
- 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.
gentle tips
- Take a pregnancy test if your period is late — your body is producing HCG now.
- Schedule your first prenatal appointment for around week 8.
- Cut alcohol completely if you haven't already.
- Add folate-rich foods (lentils, spinach, fortified grains) to a daily meal.
- Note the first day of your last period for your provider.
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
what to expect next
The heart starts beating next week — a major early milestone. Pregnancy symptoms may begin showing up soon.
common questions at week 4
How big is the baby at 4 weeks?
At 4 weeks, the baby measures about 0.1 cm and weighs approximately 0.04 g (0.1 cm (0.04") · 0.04 g (0.00 oz)).
What is developing at 4 weeks?
A tiny disc of three layers. Neural tube begins forming — it will become the brain and spinal cord. Key milestones this week include: Implantation completes; Neural tube begins forming; Heart starts as a tube; Placenta and amniotic sac form.
What symptoms are common at 4 weeks pregnant?
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.
What should I do at 4 weeks pregnant?
The heart starts beating next week — a major early milestone. Take a pregnancy test if your period is late — your body is producing HCG now. Schedule your first prenatal appointment for around week 8. Cut alcohol completely if you haven't already.
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.
helpful tools
- Due Date Calculator
Find your estimated due date from your last period or conception date.
- Baby Names
Start exploring names — an early, joyful ritual.
Medically aligned with guidance from WHO, NHS and ACOG. How we write.