Pregnancy Conception Calculator

Accuracy depends entirely on the inputs. If you know your last menstrual period exactly and have a regular cycle, the estimated conception date is typically within ±3 days of the true date. If you calculate from a clinical due date that came from an early ultrasound (best before 12 weeks), accuracy is similar. With irregular cycles or unknown LMP, the estimate becomes much less reliable — an early ultrasound is the gold standard.

Because most women know when their last period started but very few know exactly when ovulation or conception happened. LMP is a simple, objective marker that the patient herself can report. Dating from LMP is why "you're 4 weeks pregnant" can apply to someone who only conceived 2 weeks earlier — the first 2 weeks of "pregnancy" by convention happen before conception.

The standard rule is: conception ≈ due date − 266 days (for a 28-day cycle). That works out because pregnancy is dated 280 days from LMP and ovulation happens ~14 days after LMP. The calculator first works back from EDD to LMP, applies your cycle length to find ovulation, and reports that as your most likely conception day.

Sperm can survive up to 5 days in the female reproductive tract, and the egg is viable for about 12–24 hours after release. The combined fertile window therefore spans roughly the 5 days before ovulation plus ovulation day itself. Conception is most likely 2 days before ovulation through 1 day after. Our calculator shows a ±3 day window to reflect this real range.

Yes — significantly. Cycle length almost always varies in the first half of the cycle (the time from period to ovulation), while the luteal phase from ovulation to the next period stays close to 14 days. So a 21-day cycle puts ovulation around day 7, a 28-day cycle around day 14, and a 35-day cycle around day 21. The calculator adjusts conception accordingly.

For irregular cycles, an LMP-based estimate has wide error bars. Use the calculator's window output, not just the single date. For more precision, request an early dating ultrasound in the first trimester — measurements of the embryo's crown-rump length give a conception estimate accurate to within ±5 days regardless of cycle regularity.

For IVF, the conception date is precisely known — it's the egg-retrieval date. If your due date came from an IVF clinic, our LMP estimate may be off by a few days because IVF clinics often set EDD using transfer date plus a fixed offset, not Naegele's rule. Always defer to dates given by your fertility specialist.

It can provide a useful estimate but is not legal proof. The ±3-day window can stretch wider with irregular cycles, and sperm survival adds more uncertainty. For legal paternity questions, a DNA test is the only definitive answer; for medical paternity context, an early ultrasound usually gives a tighter conception window than the calculator.

Because gestational weeks count from LMP, you're typically "two weeks pregnant" before conception even happens. By the time most pregnancy tests turn positive (around week 4–5 gestational age), the embryo has only existed for 2–3 weeks. This is normal and expected — every clinical week count works the same way.

The most reliable pregnancy dating method is a first-trimester ultrasound (ideally between 8 and 13 weeks). Crown-rump length measurements at that stage are accurate to within about ±5 days. After 13 weeks, dating accuracy drops because babies start growing at slightly different rates.

No. Conception (fertilisation) happens in the fallopian tube within a day of ovulation. The fertilised egg then takes 6–12 days to travel down and implant in the uterine wall. "Implantation bleeding" — light spotting some women notice a few days before a missed period — happens roughly 6–10 days after conception.

This calculator is for personal information, not medical care. Once pregnancy is confirmed, see an obstetric provider in the first trimester — they will use ultrasound to refine dating and check fetal development. If you have questions about the conception date for medical, legal, or personal reasons, your provider can give the most accurate assessment based on multiple data points.