4 Weeks Pregnant
You’re 4 weeks pregnant — in the first trimester, and 36 weeks to go until your due date. Your baby is about the size of a poppy seed this week.
★ Implantation completes — a positive test is now possible.
How big is your baby at 4 weeks?
Size of a
poppy seed
Length
~0.1 cm
Weight
—
Your baby is holding around the size of a poppy seed this week as it fills out and gains weight rather than length.
Sizes are averages from standard fetal-growth references. Every baby grows at its own pace — your provider tracks yours at scans.
Baby development at week 4
The blastocyst implants into the uterine lining and officially becomes an embryo. It now has two layers that will form all the body's organs and tissues, plus the beginnings of the placenta and amniotic sac. The pregnancy hormone hCG rises quickly now — enough that a sensitive home test taken around your missed period can pick it up. The embryo is about the size of a poppy seed.
Your body at 4 weeks
Common symptoms many people notice around week 4 include:
- A missed period (often the first clue)
- Light implantation spotting for some
- Tender, swollen breasts
- Mild cramping and fatigue
Symptoms vary enormously from person to person and pregnancy to pregnancy. Not having a symptom on this list is just as normal as having it.
Tips for week 4
- Take a home pregnancy test from the first day of your missed period
- Book your first prenatal appointment once you get a positive
- Keep up folic acid and avoid alcohol, smoking, and raw/unpasteurised foods
What trimester is 4 weeks?
Week 4 falls in the first trimester. As a quick map: the first trimester runs weeks 1–13, the second covers weeks 14–27, and the third is week 28 through birth (a due date is set at 40 weeks). Want your own dates? Our Pregnancy Due Date Calculator shows your due date and exactly how far along you are today.
The first trimester (weeks 1–13) is when the baby's major organs and body systems form, and when pregnancy symptoms like nausea and fatigue are usually strongest. It's also when most prenatal care begins — the dating scan, early bloodwork, and (if you choose) genetic screening all happen in this window.
Frequently asked questions
- How many months is 4 weeks pregnant?
- 4 weeks pregnant is about 1 month. Pregnancy months don't divide evenly into 4-week blocks, so week-to-month conversions are approximate — most providers track pregnancy by week, not month.
- How big is the baby at 4 weeks?
- At 4 weeks, the baby is about the size of a poppy seed, roughly 0.1 cm. These are averages — healthy babies vary, and your provider tracks your baby's own growth curve at scans.
- What trimester is 4 weeks?
- Week 4 is in the first trimester. The first trimester is weeks 1–13, the second is weeks 14–27, and the third is weeks 28 to birth (around week 40).