Conception Date Calculator

Formula

How to use it

1. Choose whether you know your due date or the first day of your last period.
2. Enter the date, and for a last period your average cycle length.
3. Read the most likely conception date and the fertile window around it.

Worked examples

How the estimate works

Conception happens within about a day of ovulation, so the calculator estimates the day of ovulation. A due date is set 280 days after the last period, and ovulation in a textbook 28-day cycle comes 14 days after the period starts, leaving 266 days from conception to the due date. From a last period, ovulation is placed 14 days before the next expected period.

Sperm can survive in the body for several days, so the intercourse that led to a pregnancy may have happened up to about five days before ovulation. In a well-known study of couples trying to conceive, pregnancies came only from intercourse during the six days ending on the day of ovulation. That is the fertile window shown here.

The real day can differ by several days. Ovulation moves from cycle to cycle even in regular cycles, and a due date from an early ultrasound is itself accurate only to within about a week. A conception date from this calculator cannot establish paternity or the exact day a pregnancy began.

With irregular cycles, an estimate from a last period is unreliable; use the due date from your scan instead. After IVF, the transfer date gives the timing exactly, so this calculator is not needed.

When to talk to a doctor

If you are pregnant and unsure of your dates, your midwife or doctor can date the pregnancy with an ultrasound. If you have been trying to conceive for a year (or six months if you are 35 or older) without success, it is reasonable to ask a doctor about a fertility check. From 40, ask for a fertility assessment straight away rather than waiting (ACOG/ASRM).

See a doctor before the 6 to 12 months if you have irregular or absent periods, known or suspected advanced endometriosis, known or suspected uterine or tubal disease, or a known or suspected sperm problem.

This is an estimate for general information, not medical advice. Talk to a doctor or other qualified clinician before making health decisions based on it.

Sources

Due date = first day of last period + 280 days, and dating by first-trimester ultrasound: ACOG Committee Opinion No. 700, “Methods for Estimating the Due Date” (2017).

Six-day fertile window ending on the day of ovulation: Wilcox AJ, Weinberg CR, Baird DD. “Timing of sexual intercourse in relation to ovulation.” New England Journal of Medicine 1995;333:1517–1521.

Variability of the fertile window between and within women: Wilcox AJ, Dunson D, Baird DD. “The timing of the ‘fertile window’ in the menstrual cycle: day specific estimates from a prospective study.” BMJ 2000;321:1259–1262.

Evaluation after 12 months of trying, 6 months from age 35, straight away from 40, and earlier for known risk factors: ACOG and the American Society for Reproductive Medicine, Committee Opinion “Infertility Workup for the Women’s Health Specialist” (No. 781, 2019).