How pregnancy weeks are counted
Gestational age is counted from the first day of the last menstrual period, not from conception. That means the clock starts roughly two weeks before the pregnancy physically exists: ovulation and conception happen around the end of “week 2,” and by the time a period is missed you are already about 4 weeks pregnant. It surprises almost everyone, but the convention survives because the period is the one date people can reliably name, while ovulation is usually invisible.
The calculator accepts whichever anchor you have. A last period is used directly; a due date is walked back 280 days; a known conception date is walked back 14 days to the equivalent period start. All three converge on the same week count. If you don’t have a due date yet, our due date calculator produces one from the same inputs — and explains why it’s a window, not a day.
The three trimesters at a glance
| Trimester | Weeks | What defines it |
|---|---|---|
| First | 0 – 13 | All major organs form; nausea and fatigue peak; miscarriage risk is highest early and falls steadily |
| Second | 14 – 27 | Usually the most comfortable stretch; anatomy scan; movement becomes noticeable around 18–22 weeks |
| Third | 28 – 40+ | Rapid weight gain for the baby; lungs mature; appointments become more frequent until delivery |
What the milestones mean
First heartbeat — ~6 weeks
Cardiac activity usually becomes visible on a transvaginal ultrasound around six weeks. Seeing it matters statistically: once a heartbeat is confirmed, the chance of miscarriage drops substantially.
End of the first trimester — 13 weeks
By now every major organ system has formed; the rest of pregnancy is mostly growth and maturation. This is the point where many people share the news, because the background risk has fallen to a few percent.
Anatomy scan — ~20 weeks
The detailed structural ultrasound, typically at 18–22 weeks, checks the baby’s organs one by one and measures growth against the dating set in the first trimester.
Viability — 24 weeks
The threshold where survival outside the womb becomes possible with neonatal intensive care. Every week gained after it improves outcomes more than almost anything else in obstetrics.
Full term — 39 weeks
Babies born between 39w0 and 40w6 have the best average outcomes, which is why nothing elective is scheduled earlier. From here, labor is welcome whenever it starts.
How big is the baby? The fruit scale
Sizes vary — these are typical averages by week, using the comparison scale everyone secretly enjoys:
| Week | About the size of | Length | Weight |
|---|---|---|---|
| 8 | a raspberry | 1.6 cm | ~1 g |
| 12 | a lime | 5.4 cm | ~14 g |
| 16 | an avocado | 12 cm | ~100 g |
| 20 | a banana | 25 cm | ~300 g |
| 24 | an ear of corn | 30 cm | ~600 g |
| 28 | an eggplant | 37 cm | ~1.0 kg |
| 32 | a butternut squash | 42 cm | ~1.7 kg |
| 36 | a head of romaine | 47 cm | ~2.6 kg |
| 40 | a small watermelon | 51 cm | ~3.4 kg |
From about 28 weeks the baby roughly triples in weight — which is also when maternal weight gain accelerates. The pregnancy weight gain calculator shows the recommended curve for your build across those weeks.
As always: this page estimates and orients, it doesn’t diagnose. Your provider’s dating — especially from a first-trimester ultrasound — is the official clock, and any concerns about milestones belong in an appointment, not a calculator.