Find your conception date from a due date, last period or birthday
This conception calculator works backwards to the day a pregnancy most likely began. Start from a due date given by a midwife or scan, from the first day of the last period, or from a date of birth — the “when was I conceived?” question. It shows the estimated conception date, the fertile window of about a week in which the sex that led to the pregnancy probably happened, and a calendar of those days.
The rules are the same ones clinics use to date a pregnancy, just run in reverse. Everything is calculated in your browser, and nothing you enter is stored or sent anywhere.
How to calculate a conception date
- Choose what you know: Due date, Last period or Birth date.
- Pick the date. For the last-period method also enter your usual cycle length.
- For a birth date, add the gestation at birth if you know it (for example 39 weeks 2 days from the birth record); otherwise leave it empty.
- Read the estimated conception date, the fertile window and the conception month, and see them on the calendar.
- Press Copy result to save the dates.
Conception Date Calculator features
Three ways to work back
From the due date, the last menstrual period with any cycle length from 20 to 45 days, or a date of birth.
Conception calculator by birthday
Uses the gestation at birth when you know it, or a full-term range of 37 to 42 weeks when you do not.
Fertile window
Highlights the five days before ovulation, ovulation day and the day after, with the three most fertile days.
Calendar view
Whole weeks around the window so you can match the dates to events, trips or diaries.
Honest limits
States plainly what dates can and cannot tell you, including about paternity.
How the conception date is worked out
Pregnancy is dated from the first day of the last menstrual period (LMP): the due date is LMP + 280 days (40 weeks), as set out by the American College of Obstetricians and Gynecologists in Committee Opinion 700, Methods for Estimating the Due Date (2017). In a 28-day cycle, ovulation — and so conception — happens about 14 days after the period starts, which makes the due date conception + 266 days. Reversed: conception ≈ due date − 266 days. With longer or shorter cycles ovulation moves, so the calculator uses LMP + cycle length − 14.
From a birth date: gestational age at birth minus two weeks gives the time since conception. A baby born on 1 January 2000 at 40 weeks was probably conceived around 10 April 1999. If the gestation is unknown, the range for a full-term birth (37 to 42 weeks) puts conception between 27 March and 1 May 1999.
| You know | Conception estimate |
|---|---|
| Due date | Due date − 266 days |
| First day of last period | LMP + cycle length − 14 days |
| Birth date and gestation | Birth − gestation + 14 days |
| Birth date only | Birth − 266 days (range 245–280 days) |
Why it is a window, not a single day
Sperm can survive in the reproductive tract for up to about five days, while an egg lives only about a day after ovulation. In a classic study of couples trying to conceive (Wilcox, Weinberg and Baird, New England Journal of Medicine, 1995), every pregnancy came from sex in the six days ending on ovulation day, most often the two days before. So the sex that led to a pregnancy could have happened up to five days before conception itself.
Ovulation also varies from cycle to cycle, and pregnancies vary in length: in women whose exact day of ovulation was measured, pregnancy length still ranged over 37 days (Jukic and colleagues, Human Reproduction, 2013). A due date from an early ultrasound is more reliable than one from a remembered period, and conception estimated from a birth date is the least precise of all.
What a conception date cannot tell you
- Paternity. When two possible fathers are involved, dates rarely separate them: the window is about a week wide and the estimate itself can be out by a week or more. Only a DNA paternity test, which can be done during pregnancy or after birth, establishes paternity.
- IVF pregnancies. The transfer date and embryo age give an exact date; use the pregnancy due date calculator with the IVF option.
- Medical decisions. These are estimates for information only, not a diagnosis. Ask a doctor or midwife about dating scans or anything that worries you. To plan a future pregnancy, the ovulation calculator shows upcoming fertile days.
Conception Date Calculator FAQ
How do I calculate my conception date from my due date?
Subtract 266 days (38 weeks) from the due date. The conception calculator above does this and also shows the fertile window around it, because the sex that led to the pregnancy could have been up to five days earlier.
When was I conceived, based on my birthday?
Choose Birth date and enter your birthday. If you were born at full term, conception was most likely about 266 days earlier; if your birth record gives the gestation, enter it for a closer estimate.
How many months is conception from birth?
About 8 calendar months and 3 weeks for a baby born at 40 weeks — 266 days. A due date in March usually means conception in June of the year before.
Can a conception calculator tell who the father is?
No. The possible window is about a week, and the estimate can be out by more than that. Only a DNA paternity test can establish paternity.
How accurate is a conception date calculator?
It is as accurate as the date you start from. Early-scan due dates are best; period dates can be out by a week or more, and birth dates by several weeks if the gestation is unknown.