First trimester · 27 weeks to go
| Fact | Week 13 |
|---|---|
| Days of pregnancy | 85–91 since LMP |
| Gestational age span | 12w0d to 12w6d |
| Trimester | First trimester (weeks 1–13) |
| Pregnancy month | Month 1 (weeks 1–4) |
| Average length | 2.9 in (crown to rump) |
| Average weight | 0.81 oz |
| Size comparison | a peach |
| Weeks remaining | 27 (40-week due date) |
Week 13 is the last week of the first trimester, and the odds now favor a healthy outcome by a wide margin. Your baby has vocal cords forming, and unique fingerprints are finishing on fingertips smaller than rice grains.
The spleen and liver share blood-cell duty, and urine output ramps up. Insulin production begins, bones of the arms and legs start hardening from cartilage, and the head slows its growth rate so the body can catch up.
Gestationally, week 13 means you are 12 weeks and 0 to 6 days pregnant, day 85 to day 91 counted from the first day of your last period. This is week 13 of the first trimester (weeks 1–13). Prenatal appointments run about every four weeks in this stretch. Week 14 brings the second trimester. 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 13.
Start budgeting for maternity clothes; the transition happens fast in the next month. A photo timeline from here makes a keepsake later.
Weeks of pregnancy don't map cleanly to calendar months, but the common division puts week 13 falls in month 1 of pregnancy (weeks 1–4). Week 13 is part of the first trimester, which runs from week 1 to week 13. Gestationally, week 13 means you are 12 weeks and 0 to 6 days pregnant.
At week 13 the baby averages 2.9 in long and weighs about 0.81 oz, roughly the size of a peach. Sizes come from fetal growth charts (crown-rump length before week 20), and normal variation week to week is wide; your ultrasound measurements always outrank a chart.