Enter the first day of your last period. We'll tell you exactly how far along you are, what trimester, what's developing this week, and how many days until your due date.
Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. That's the obstetric standard, and it means you're already counted as a few weeks pregnant around the time fertilization actually happens — fertilization typically occurs around week 3, when the fertilized cell is dividing as it travels down the fallopian tube, roughly two weeks after the LMP date you enter above.
Cycle length matters because ovulation doesn't land on the same calendar day for everyone — in a normal cycle it can happen anywhere between day 11 and day 21. Most cycles run 28 days, but if yours is shorter or longer, this calculator adjusts using the standard formula: it adds the difference between your cycle length and 28 to the LMP-derived dates before working out your current week and due date.
The three trimesters break down by week. The first trimester runs through week 12 — miscarriage risk drops significantly after week 10, since most pregnancy loss happens before then, and NIPT genetic testing is typically offered around week 12. The second trimester runs from week 13 to week 26: week 20 is the halfway point, when an anatomy ultrasound is typically scheduled, and weeks 23–24 mark the viability milestone, when survival outside the uterus becomes possible with intensive NICU support. The third trimester starts at week 27 and continues to birth; week 37 is considered full term.
Your due date is calculated as 280 days (40 weeks) from your cycle-adjusted LMP date. Treat it as an estimate, not a deadline — only about 4% of babies are born exactly on their due date, and most arrive within two weeks of it either way. By week 41 your provider will likely start discussing induction, and induction is almost always recommended by week 42 if labor hasn't started on its own.
LMP-based dating is accurate to roughly ±2 weeks, precisely because ovulation timing varies the way described above. A first-trimester ultrasound (week 8–13) is more precise, accurate to about ±5 days — if you've had a dating ultrasound and its date differs from what this calculator shows, your provider's ultrasound-based date is the one to use.
Source: ACOG Committee Opinion 700, "Methods for Estimating the Due Date" (2017).