How due dates are calculated
Pregnancy is counted as 40 weeks (280 days) from the first day of your last menstrual period — Naegele's rule — even though conception happens about two weeks in. The calculator adjusts for your cycle length, because ovulation comes later in longer cycles: with a 32-day cycle, the due date shifts 4 days later than the standard rule.
Conception: due = conception + 266 days
IVF: due = transfer + 263 (day-3) or + 261 days (day-5)
Trimesters and milestones
| Week | Milestone |
|---|---|
| Week 4–5 | Positive home test; embryo implants |
| Week 12 | End of 1st trimester; miscarriage risk drops sharply |
| Week 20 | Anatomy scan; halfway point |
| Week 24 | Viability milestone |
| Week 28 | 3rd trimester begins |
| Week 37 | Early term — baby is considered full-grown soon after |
| Week 39–40 | Full term; due date |
| Week 42 | Post-term; induction usually discussed |
How accurate is a due date?
Only about 4–5% of babies arrive on their exact due date; roughly 90% are born within two weeks either side. A first-trimester ultrasound (which measures the embryo directly) dates pregnancy more precisely than any calendar rule and will override the LMP estimate if they differ by more than a few days — so treat this date as the center of a window, not an appointment.
This tool provides general estimates for educational purposes and is not medical advice. Consult a healthcare professional for guidance specific to you.
Frequently asked questions
How many weeks pregnant am I?
Count from the first day of your last menstrual period — not from conception. If your last period started 10 weeks ago, you are 10 weeks pregnant, even though the embryo is about 8 weeks old. This calculator shows your current week automatically.
Why does the doctor's date differ from my calculation?
Clinics prioritize early ultrasound measurements over calendar math. If your first-trimester scan dated the pregnancy differently by more than about 5–7 days, the scan date becomes the official one.
Can my due date change during pregnancy?
The official estimate is usually set once, at the first ultrasound, and kept. Later scans measure growth, which varies too much between babies to re-date the pregnancy reliably.
What if I don't remember my last period?
Use the conception option if you know it (for example from ovulation tracking), or simply wait for the dating ultrasound — it's the standard solution and accurate to within about 5 days in the first trimester.
When will I likely actually give birth?
First babies arrive a median of about 40 weeks + 3–5 days; subsequent babies slightly earlier. Statistically, you're more likely to deliver in the week after your due date than the week before — pack the bag by week 36 anyway.