Conception Date Calculator

Worked example

A due date of 15 April 2027 puts the last period on 9 July 2026 and conception around 23 July — most likely between 18 and 24 July, because sperm survive up to five days and the egg about one. Implantation follows a week or so later, and a test is usually reliable from about 6 August.

How to work it out yourself

1. 1.Work back from the due date if you have had a dating scan: 280 days to the last period, then forward to ovulation. A scan-based due date is more accurate than a remembered period, and this inherits that accuracy.
2. 2.Adjust the cycle length. Ovulation is about 14 days before the next period rather than 14 days after the last one, so on a 32-day cycle conception is four days later than the textbook assumption.
3. 3.Read the window rather than the day. Sperm survive up to five days, the egg about one, so intercourse anywhere in that span can be the conception — which is why a single date can never settle paternity.

How the cycle length moves conception

From the same due date. A longer cycle means the period was earlier relative to conception, not that the pregnancy is older.

The formula

1. First day of the last period2027-04-15 - (280 + 28 - 28)
2. Most likely date of conception2026-07-09 + 28 - 14
3. Earliest likely2026-07-23 - 5
4. Latest likely2026-07-23 + 1
5. Estimated due date2026-07-09 + 280 + 28 - 28
6. Implantation window opens2026-07-23 + 6
7. Implantation window closes2026-07-23 + 12
8. A test is usually reliable from2026-07-23 + 14
9. Days the window spans

Source: ACOG — methods for estimating the due date (Committee Opinion 700), NIH — sperm survival and the fertile window (Wilcox et al., NEJM 1995)

Questions people actually ask

Can a conception date be exact?

No. Sperm survive in the reproductive tract for up to five days and the egg for about one, so conception can follow intercourse by anything from a few hours to five days. The date above is the middle of that window, not a fact.

Why does the pregnancy count from before conception?

Because gestational age is counted from the first day of the last period, which is about two weeks before conception on a typical cycle. It is why you are called four weeks pregnant when the embryo is two weeks old, and why a 40-week pregnancy is really about 38 weeks of gestation.

Does a longer cycle change the due date?

Yes, and most calculators ignore it. Ovulation is about 14 days before the next period, so on a 35-day cycle it happens on day 21 rather than day 14 and the due date moves a week later. This page applies that correction in both directions.

Related

• Due Date CalculatorEstimated due date from a last menstrual period or a known conception date, with the current gestational age.
• Ovulation CalculatorOvulation day and the six-day fertile window from your last period, counted back from the next period rather than forward from day 14.
• Period CalculatorYour next six periods from the last one, with the prediction window widening the further out it goes — because cycle length varies month to month.
• Pregnancy Weight Gain CalculatorWhere your weight should be at this point in pregnancy, from the 2009 Institute of Medicine ranges for your pre-pregnancy BMI.
• Dog Pregnancy CalculatorWhelping date from the mating date, on the 63-day canine gestation, with the dates each check and each stage of the pregnancy falls due.

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