3 Weeks Pregnant
You’re 3 weeks pregnant — in the first trimester, and 37 weeks to go until your due date. Your baby is about the size of a poppy seed this week.
★ Fertilisation occurs — the baby's genetic blueprint is set.
How big is your baby at 3 weeks?
Size of a
poppy seed
Length
~0.01 cm
Weight
—
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 3
Conception happens this week. A single sperm fertilises the egg, forming a one-cell zygote that immediately begins dividing as it travels down the fallopian tube. By the end of the week it's a tiny ball of cells called a blastocyst, smaller than the period at the end of this sentence. The genetic blueprint — including biological sex — is already set at the moment of fertilisation.
Your body at 3 weeks
Common symptoms many people notice around week 3 include:
- Usually none yet — it's very early
- Some notice light implantation-related changes toward the end
- The hormone hCG is just beginning to be produced
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 3
- Continue your prenatal vitamin with at least 400mcg folic acid
- Avoid alcohol and limit caffeine from here on
- It's too early for a positive pregnancy test — wait about a week
What trimester is 3 weeks?
Week 3 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 3 weeks pregnant?
- 3 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 3 weeks?
- At 3 weeks, the baby is about the size of a poppy seed, roughly 0.01 cm. These are averages — healthy babies vary, and your provider tracks your baby's own growth curve at scans.
- What trimester is 3 weeks?
- Week 3 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).