First trimester · 36 weeks to go
| Fact | Week 4 |
|---|---|
| Days of pregnancy | 22–28 since LMP |
| Gestational age span | 3w0d to 3w6d |
| Trimester | First trimester (weeks 1–13) |
| Pregnancy month | Month 1 (weeks 1–4) |
| Average length | 0.04 in (crown to rump) |
| Average weight | not yet measurable |
| Size comparison | a poppy seed |
| Weeks remaining | 36 (40-week due date) |
Week 4 is when the pregnancy becomes detectable. The blastocyst finishes implanting in the uterine lining, and the cells that will become the placenta start pumping out hCG, the hormone a home test measures. A sensitive test can turn positive now.
The embryo settles into two layers: the epiblast, which becomes your baby, and the hypoblast, which feeds it early on. The amniotic sac and yolk sac are forming. Size is still measured in millimeters, about a tenth of a centimeter.
Gestationally, week 4 means you are 3 weeks and 0 to 6 days pregnant, day 22 to day 28 counted from the first day of your last period. This is week 4 of the first trimester (weeks 1–13). Prenatal appointments run about every four weeks in this stretch. Week 6 brings the first heartbeat, usually visible on ultrasound. 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 4.
Take a home pregnancy test if your period is late; first morning urine holds the most hCG. Call to schedule your first prenatal visit, usually set for around week 8.
Weeks of pregnancy don't map cleanly to calendar months, but the common division puts week 4 falls in month 1 of pregnancy (weeks 1–4). Week 4 is part of the first trimester, which runs from week 1 to week 13. Gestationally, week 4 means you are 3 weeks and 0 to 6 days pregnant.
At week 4 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.