First trimester · 35 weeks to go
| Fact | Week 5 |
|---|---|
| Days of pregnancy | 29–35 since LMP |
| Gestational age span | 4w0d to 4w6d |
| Trimester | First trimester (weeks 1–13) |
| Pregnancy month | Month 1 (weeks 1–4) |
| Average length | 0.09 in (crown to rump) |
| Average weight | not yet measurable |
| Size comparison | a sesame seed |
| Weeks remaining | 35 (40-week due date) |
Week 5 begins the embryonic period proper, the stretch when every major organ starts taking shape. The heart is forming and will begin beating in the next few days, and the neural tube, future brain and spine, is folding closed.
Three layers now divide the work: the ectoderm builds brain, spine, and skin; the mesoderm builds heart, muscles, and bones; the endoderm builds lungs, gut, and bladder. The heart tube starts pulsing around day 21 to 22 after conception.
Gestationally, week 5 means you are 4 weeks and 0 to 6 days pregnant, day 29 to day 35 counted from the first day of your last period. This is week 5 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 5.
Book your first prenatal appointment now and be ready with your LMP date and medication list. If you haven't started a prenatal vitamin with folic acid, today beats tomorrow.
Weeks of pregnancy don't map cleanly to calendar months, but the common division puts week 5 falls in month 1 of pregnancy (weeks 1–4). Week 5 is part of the first trimester, which runs from week 1 to week 13. Gestationally, week 5 means you are 4 weeks and 0 to 6 days pregnant.
At week 5 there isn't much to measure yet; the baby is on the scale of a sesame seed. Measurable length in inches begins with the embryo stage in the next few weeks.