First trimester · 39 weeks to go
| Fact | Week 1 |
|---|---|
| Days of pregnancy | 1–7 since LMP |
| Gestational age span | 0w0d to 0w6d |
| Trimester | First trimester (weeks 1–13) |
| Pregnancy month | Month 1 (weeks 1–4) |
| Average length | not yet measurable (too small to measure) |
| Average weight | not yet measurable |
| Size comparison | not yet conceived |
| Weeks remaining | 39 (40-week due date) |
Week 1 starts on the first day of your last period, which means you're not pregnant yet in any biological sense. Doctors count from here because the LMP is the one date most people can pinpoint. Conception is still about two weeks away.
Your body is shedding last cycle's uterine lining and starting fresh. Follicles in your ovaries are beginning to mature, one of which will release next cycle's egg. The pregnancy count that started this week will run 280 days to your due date.
Gestationally, week 1 means you are 0 weeks and 0 to 6 days pregnant, day 1 to day 7 counted from the first day of your last period. This is week 1 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 1.
Start a prenatal vitamin with 400–600 mcg folic acid now if you're trying; the neural tube closes in the first month, often before a positive test.
Weeks of pregnancy don't map cleanly to calendar months, but the common division puts week 1 falls in month 1 of pregnancy (weeks 1–4). Week 1 is part of the first trimester, which runs from week 1 to week 13. Gestationally, week 1 means you are 0 weeks and 0 to 6 days pregnant.
At week 1 there isn't much to measure yet; the baby is on the scale of not yet conceived. Measurable length in inches begins with the embryo stage in the next few weeks.