First trimester · 37 weeks to go
| Fact | Week 3 |
|---|---|
| Days of pregnancy | 15–21 since LMP |
| Gestational age span | 2w0d to 2w6d |
| Trimester | First trimester (weeks 1–13) |
| Pregnancy month | Month 1 (weeks 1–4) |
| Average length | 0.01 in (too small to measure) |
| Average weight | not yet measurable |
| Size comparison | a poppy seed |
| Weeks remaining | 37 (40-week due date) |
Week 3 is conception week. Sperm meets egg in the fallopian tube, and the fused cell begins dividing as it drifts toward the uterus. Your baby is a microscopic ball of cells this week, already carrying a complete genetic blueprint.
The fertilized egg becomes a zygote, then a morula, then a blastocyst as it travels. Around the end of the week it reaches the uterus and begins burrowing into the plush lining, a process called implantation that finishes early in week 4.
Gestationally, week 3 means you are 2 weeks and 0 to 6 days pregnant, day 15 to day 21 counted from the first day of your last period. This is week 3 of the first trimester (weeks 1–13). Prenatal appointments run about every four weeks in this stretch. Week 4 brings a positive home pregnancy test as hCG rises. If your cycle runs longer or shorter than the 28 days the due-date math assumes, your ovulation date and due date shift with it, which is why a first-trimester ultrasound remains the gold standard for dating. Every number on this page is an average across pregnancies; your own scan dates and your baby's measurements are what actually describe your week 3.
Avoid alcohol, smoking, and high-mercury fish from now on; the neural tube and earliest organs form in the next two weeks, when the embryo is most sensitive to harm.
Weeks of pregnancy don't map cleanly to calendar months, but the common division puts week 3 falls in month 1 of pregnancy (weeks 1–4). Week 3 is part of the first trimester, which runs from week 1 to week 13. Gestationally, week 3 means you are 2 weeks and 0 to 6 days pregnant.
At week 3 there isn't much to measure yet; the baby is on the scale of a poppy seed. Measurable length in inches begins with the embryo stage in the next few weeks.