How to use the conception calculator
- Choose whether to count back from a due date or a birth date.
- Enter the due date, or the date of birth.
- For a birth date, enter the length of pregnancy at birth in weeks and days. It is usually in the birth record. Use 40 weeks if you don't know it.
- Choose how the due date was set. Most pregnancies are dated by an ultrasound before 14 weeks.
- Read the most likely conception date, the likely range around it, and the table.
How the conception date is worked out
Pregnancy is counted from the first day of the last period, and conception usually happens about 14 days later. So a due date, which marks 40 weeks (280 days), falls about 266 days after conception. For a birth, take the length of the pregnancy at birth and subtract those first two weeks.
The likely range comes from the accuracy of ultrasound dating given by the American College of Obstetricians and Gynecologists (ACOG, Committee Opinion 700). An early scan that measures the baby's crown-rump length is accurate to about 5 to 7 days. Scans at 14 to 21 weeks are accurate to about 7 to 10 days, and later scans are less precise. The calculator uses the upper figure for each.
Worked example
The calculator opens with a due date 120 days from today, set by an ultrasound before 14 weeks. On October 29, 2026, that due date is February 26, 2027.
- Conception: February 26, 2027 − 266 days = June 5, 2026
- Likely range at ±7 days: May 29 to June 12, 2026
- If conception was on June 5, sex from May 31 to June 6 could have led to it
- Equivalent first day of the last period: February 26, 2027 − 280 days = May 22, 2026
If the due date had come from a scan at 14 to 21 weeks instead, the likely range would widen to May 26 to June 15, 2026.
Why there is always a range
Three things make an exact answer impossible:
- Dating error. Every due date is an estimate, even from an ultrasound. Babies of the same age vary a little in size, so a scan can be off by several days.
- Ovulation timing. The 266-day rule assumes ovulation 14 days after the last period started. In real cycles it can come several days earlier or later.
- Sperm survival. Sperm can survive in the body for up to 5 days, so the day of sex and the day of conception can be different days.
Counting back from a birth adds a fourth: babies are rarely born exactly on their due date. A baby born 10 days early or late shifts the conception estimate by the same 10 days unless you use the real length of the pregnancy. That is why the birth-date option asks for the weeks and days at birth.
| Born at | Conception, for a birth on Oct 1, 2026 |
|---|---|
| 38 weeks | Jan 22, 2026 |
| 39 weeks | Jan 15, 2026 |
| 40 weeks | Jan 8, 2026 |
| 41 weeks | Jan 1, 2026 |
For planning dates within a current pregnancy, such as implantation or when to test, see the pregnancy conception calculator.
Frequently asked questions
Can this tell who the father is?
No. A calculated conception date has a range of at least a week or two, and sex days apart can lead to the same conception. Only a DNA paternity test can establish who the father is.
Is it more accurate to use the due date or the birth date?
Usually the due date, if it was set by an early ultrasound. A birth date only works well if you know how many weeks pregnant the mother was at the birth, because that varies by several weeks between babies.
What if the baby was conceived by IVF?
Then the conception date is known from the clinic records: fertilization took place in the lab 3 or 5 days before the embryo transfer. The calculator is meant for natural conception, where the date has to be estimated.
Why does my doctor's timeline seem two weeks off?
Doctors count pregnancy from the first day of the last period, about two weeks before conception. So a baby conceived on June 5 is described as 2 weeks along that day. Ask your doctor or midwife if you want to go over your own dates.
Related calculators
- Pregnancy Conception Calculator — Estimate your conception date and fertile window from your last period or due date, with the early pregnancy timeline.
- Due Date Calculator — Your estimated due date by Naegele's rule, adjusted for cycle length, or from a conception or IVF transfer date.
- Pregnancy Calculator — How many weeks pregnant you are, your due date, trimester and key milestones, from your last period, conception, ultrasound or IVF.
Results are estimates for general information and are not medical advice. Talk to a doctor or other qualified professional about your health. See our disclaimer.