Conception date from LMP, due dates from LMP, conception, or IVF transfer, the fertile window, gestational age today, trimester stage, countdown, IOM weight-gain ranges, and an ovulation predictor — one dropdown for pregnancy calendar math. Estimates only: first-trimester ultrasound dating is more accurate and belongs with your provider.
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Calendar dating, honestly explained
Naegele's rule counts 280 days (40 weeks) from the first day of the last period, assuming ovulation on day 14 of a 28-day cycle. If your cycles run 32 days, everything shifts four days later — hence the cycle-length adjustment on every date member here. Conception sits at LMP + (cycle − 28) + 14, and gestational age deliberately includes the two weeks before conception technically existed.
When you know more than the calendar
A known conception date (IVF, ovulation kits, careful timing) gives a due date of conception + 266 days. IVF is tighter still: a day-5 blastocyst transfer is already 2w5d "pregnant," so the due date is transfer + 261 days; a day-3 embryo transfer gets + 263. The fertile window around any conception date spans about six days — sperm wait up to five, the egg lasts one.
Worked example
LMP March 10, 2026, textbook 28-day cycle: conception ≈ March 24 (LMP + 14).
Due date by Naegele: March 10 + 280 days = December 15, 2026.
Cycle runs 32 days? Add 4: due December 19, conception March 28.
IVF day-5 transfer May 1, 2026: + 261 days = January 17, 2027.
How to use it
Pick the question from the dropdown — a date to find, today’s gestational age, or weight gain.
Enter the first day of the last period (not the last day) and your usual cycle length.
For IVF, use the transfer date and the embryo stage, day 3 or day 5.
Read the board — then let a first-trimester ultrasound refine every estimate.
Frequently asked questions
When did I conceive?
Conception happens on ovulation day, about 14 days before the next period — for a 28-day cycle that is LMP + 14. A 34-day cycle shifts it to LMP + 20. Because the egg survives only about a day, conception from intercourse can also happen up to five days after sex, which is why the window member shows ±2 days around the estimate.
How accurate is a due date calculated from my last period?
Reasonable, not exact: Naegele’s rule assumes a 28-day cycle with day-14 ovulation and a correctly remembered LMP. Only about 4–5% of babies arrive on the estimated date and most deliver between 37 and 41 weeks. A first-trimester ultrasound dates a pregnancy to within about a week and should override the calendar when they disagree.
Why does an IVF due date use different numbers?
The embryo’s age is known exactly, so no assumptions are needed. A day-5 blastocyst is already 19 days of gestational age (the two pre-conception weeks plus five embryo days), leaving 280 − 19 = 261 days to the due date. A day-3 embryo is 17 days, leaving 263.
Which trimester am I in?
Count completed weeks since the LMP: through 13 weeks is the first trimester, 14–27 the second, and 28 onward the third. Full term starts at 39 weeks. This page’s trimester member shows those boundary dates for your own LMP.
How much weight should I gain during pregnancy?
By pre-pregnancy BMI for a single baby (IOM): underweight 28–40 lb, normal weight 25–35 lb, overweight 15–25 lb, and obese 11–20 lb. Twins have higher targets. Your provider personalizes this, and rate matters as much as total.
Can this predict my next ovulation?
For regular cycles, yes as an estimate: next period start − 14 days, because the luteal phase is the steady half of the cycle. The predictor shows the six-day fertile window (five days before through one day after ovulation). Ovulation kits, basal temperature, and provider monitoring track it directly — and none of it is reliable as contraception. Medical decisions belong with your clinician.