Does IVF (in vitro fertilisation) change the conception date calculation?

Yes. With IVF, the exact date of egg fertilisation or embryo transfer is known. For a 5-day blastocyst transfer, the conception date is taken as the transfer date minus 5 days. Your doctor will determine the precise date based on clinic records — the results of an online calculator do not replace this assessment.

How is the conception date calculated for twins?

For twin pregnancies (both identical and fraternal), the conception date is calculated in the same way as for a single pregnancy. Identical twins result from the splitting of one fertilised egg (one conception date), while fraternal twins result from the fertilisation of two eggs in the same ovulation cycle. The length of a twin pregnancy is usually shorter — the due date is set individually by the doctor.

Does the calculator work with irregular cycles?

The calculator assumes a standard 28-day cycle with ovulation on day 14. For irregular cycles, the LMP-based method may be inaccurate. In such cases it is better to use the due-date method based on ultrasound results, or to consult a doctor to determine the exact ovulation date.

When should I consult a doctor about the conception date?

Consult your doctor or midwife if: you have irregular cycles, you have undergone fertility treatment, you are pregnant after IVF, or you want to confirm your due date by ultrasound. The most accurate way to establish the conception date and due date is a first-trimester ultrasound measuring the crown-rump length (CRL) of the foetus.

Calculator · Parenting

Conception Date Calculator 2026

Want to know when your baby was most likely conceived? Our conception date calculator lets you work it out in two ways: from your expected due date, or from the date of your last menstrual period (LMP). Simply choose the method, enter the relevant date, and the calculator will instantly estimate the fertilisation date, the due date, and the current week of pregnancy counted from conception. Method one — from the due date — works by subtracting 266 days from the provided due date. This figure corresponds to the average length of a pregnancy measured from the moment of fertilisation (38 weeks = 266 days). This is the most accurate method when the due date has already been confirmed by ultrasound. Method two — from the last menstrual period — is based on the assumption that ovulation, and therefore fertilisation, occurs around day 14 of the cycle. The calculator adds 14 days to the first day of the last period to estimate the conception date. This approach is widely used in gynaecology, although the result may be less precise for women with irregular cycles. The calculator result is an estimate — for certainty, consult your doctor or midwife, who will confirm the date by ultrasound measurement.

Reviewed byRedakcja LiczbnikUpdated: 10.06.2026Sources & methodology

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How the conception date calculator works

The calculator uses two methods based on medical standards applied in gynaecology and obstetrics. **From the due date:** The due date (established by a doctor via ultrasound or Naegele's rule) is reduced by 266 days. Why 266? Because that is how long the average pregnancy lasts from the moment of fertilisation (38 weeks = 266 days). The result is the approximate conception date. **From the last menstrual period (LMP):** 14 days are added to the first day of the last period — the standard time after which ovulation occurs in a 28-day cycle. This gives the approximate conception date. The due date is calculated as LMP + 280 days (Naegele's rule). **Weeks of pregnancy:** The calculator counts how many full weeks have elapsed from the conception date to today. Note that doctors usually count pregnancy weeks from the last menstrual period — in that convention the gestational week is about 2 weeks higher. **Date formatting:** All dates are displayed in DD.MM.YYYY format. Calculations use UTC midnight arithmetic, eliminating errors caused by daylight saving time changes.

Example: due date 1 January 2026

Suppose the due date is 1 January 2026. Select the "from due date" method and enter 2026-01-01. The calculator subtracts 266 days and returns a conception date of 11 April 2025. The due date remains unchanged — 01.01.2026. If today is 1 October 2025, then 173 days have passed since conception, which corresponds to week 24 of pregnancy counted from fertilisation. Second example: the last period started on 1 March 2025. Select the "from last menstrual period" method and enter 2025-03-01. The calculator adds 14 days and shows the conception date: 15 March 2025. The due date is 1 March 2025 + 280 days = 5 December 2025. If today is 1 June 2025, then 78 days have passed since conception — you are in week 11 of pregnancy from fertilisation.

Frequently asked questions about conception date

How do I calculate my conception date from my due date?

To calculate the conception date from the due date, subtract 266 days from the expected delivery date. This corresponds to the average length of pregnancy measured from fertilisation — 266 days (38 weeks). Our calculator performs this calculation automatically once you enter the due date.

How do I calculate my conception date from my last period?

If you know the date of your last menstrual period (LMP), add 14 days to that date. This is because ovulation and fertilisation typically occur in the middle of the cycle — around day 14 from the first day of the period. This method is widely used in gynaecology.

Is the conception date from the calculator accurate?

The calculator provides an indicative result. The exact date of fertilisation depends on individual cycle length, the length of the follicular phase, and sperm survival time (up to 5 days). For women with irregular cycles, the margin of error can be several days. The most reliable method for confirming the conception date is a first-trimester ultrasound with CRL measurement.

How many days does pregnancy last from conception to birth?

The average pregnancy lasts 266 days (38 weeks) from the date of fertilisation. However, doctors use gestational age measured from the last menstrual period (LMP), giving 280 days (40 weeks). Both measures are correct — they differ only in the starting point.

What is Naegele's rule and how does it relate to conception date?

Naegele's rule is the most commonly used method for estimating the due date: add 280 days (40 weeks) to the first day of the last menstrual period. This implies that conception (ovulation + fertilisation) occurs around day 14 after LMP, and from conception to delivery is 266 days. This rule is used by default by obstetricians and by our calculator.

How does the calculator count weeks of pregnancy?

The calculator counts the number of complete weeks that have elapsed from the calculated conception date to today. The result may differ slightly from the gestational week given by your doctor, who usually counts from the last menstrual period (LMP). The difference is typically 2 weeks.

Indicative result

The result is an estimate — please consult a doctor.

Due date calculator
Calculate your estimated due date (EDD) from your last menstrual period (LMP) using Naegele's rule. See gestational age in weeks and days until delivery — free.
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