Pregnancy Due Date Calculator

How Does the Pregnancy Calculator Work?

Our calculator uses the globally recognized medical Naegele's rule: due date = the first day of your last menstrual period + 280 days (40 weeks). This is the average length of a normal pregnancy and includes about two weeks before fertilization actually happened.

What sets our calculator apart: it shows the result in both Gregorian and Hijri, calculates your current week and day precisely, shows which trimester you are in, your pregnancy progress percentage, and your baby's approximate size this week.

What Is the Most Accurate Pregnancy Calculator?

Accuracy comes less from the calculator's design and more from the quality of the input: the first day of your last period is the single most important number in the whole equation. A calculator that asks for this date and shows its method transparently is far more accurate than one that guesses or assumes a 28-day cycle without telling you.

Calculating Pregnancy: Naegele's Rule in Detail

The German physician Franz Karl Naegele formulated this rule in the nineteenth century, and it remains to this day the primary reference used by the American College of Obstetricians and Gynecologists (ACOG) to estimate the due date before confirming it by ultrasound.

The full formula: estimated due date = first day of your last menstrual period + one year − 3 months + 7 days, which equals adding 280 days (40 full weeks). Example: last period January 1 → +7 days = January 8 → −3 months = October 8 of the previous year → +one year = October 8 of the current year.

The rule's medical assumptions: a regular 28-day menstrual cycle, ovulation on day 14 of the cycle, and an average pregnancy length of 280 days from the first day of the period (266 days from actual fertilization). When your cycle differs from 28 days, doctors adjust the result by adding or subtracting the difference.

Why do we calculate from the last period and not from the day of conception? Because the exact day of conception is hard to pin down (sperm can live up to 5 days), while the first day of your period is a known and usually documented date. The first two weeks of gestational age are a preparation phase: ovulation and fertilization happen at their end.

Worked Examples with Real Numbers

Example one: Suppose the first day of your last period was January 1, 2026. Due date = January 1, 2026 + 280 days = October 8, 2026 (a Thursday). In the Hijri calendar this corresponds to Rabi al-Thani 26, 1448 AH (approximate, ±2 days). So if today is October 7, 2026, you are in week 40 + 6 days, with only one day left until the estimated date.

Example two: last period March 15, 2026 → due date December 20, 2026 (a Sunday), corresponding in Hijri to about Rajab 11, 1448 AH. Note that the calculation adds the actual days of each month (exactly 280 days), not 9 calendar months, so it may differ by a day or two from the rough month-adding method.

How can you verify this yourself? Count 280 days from your period date using any calendar, or apply the manual Naegele formula (+7 days, −3 months, +one year), and compare the result with what the calculator gives; they must match exactly.

Calculate My Pregnancy: Common Mistakes That Ruin the Result

Mistake one: entering an inaccurate date. Many enter the day they confirmed the pregnancy or the day the period ended instead of its first day; a difference of days can shift your whole pregnancy week. The golden rule: the first day of actual bleeding is day zero.

Mistake two: ignoring your actual cycle length. The rule assumes 28 days; if your cycle is 35 days, ovulation comes a week later and the true due date shifts by the same amount. Enter your real average cycle length from 3 consecutive cycles.

Mistake three: confusing gestational age with fetal age. Gestational age (counted from the period) is about two weeks more than the actual fetal age (counted from fertilization); do not worry if the ultrasound says the baby is two weeks younger.

Mistake four: recalculating every week. Some mothers calculate from a different date each time, so the due date keeps changing and causes anxiety; fix one confirmed date and stick to it.

Mistake five: ignoring the first-trimester ultrasound. If you had an ultrasound between weeks 8 and 12 and it differs from your calculation by more than a week, the ultrasound is the medically accepted reference, not the period calculation.

The Limits of a Pregnancy Calculator and When Not to Rely on It

The calculator is an excellent estimation tool, but you should know its limits: first, only about 5 percent of women give birth exactly on the estimated date; most births happen within two weeks before or after it. Second, irregular cycles (shorter than 21 or longer than 35 days) make the rule less accurate. Third, IVF pregnancies are dated from egg retrieval or embryo transfer, not from the period. Fourth, multiple pregnancies (twins) usually end before week 40. Fifth, the result never replaces your doctor's follow-up and the regular ultrasound schedule.

When Is the Result an Estimate?

The calculation is accurate if your cycle is regular (28 days) and you remember its date precisely. But remember: only 5 percent of women give birth exactly on the estimated date; most births happen a week before or after. If your cycle is irregular or you do not remember the date, a first-trimester ultrasound (8 to 12 weeks) is the most accurate option.

The Pregnancy Journey: What Awaits You

First trimester (1-13): the baby's basic organs form; you may experience nausea and fatigue. The most important step: folic acid and your first doctor visit between weeks 8 and 10.

Second trimester (14-27): the most beautiful phase; nausea fades and you feel the baby move. The detailed anatomy scan is between weeks 18 and 22, and the glucose test between 24 and 28.

Third trimester (28-40): rapid growth and weight gain; visits every two weeks, then weekly. Pack your hospital bag from week 36.

Follow every week in detail in the pregnancy weeks guide →

Pregnancy Guides and the Pregnancy Tracker

Why Is Our Calculator More Accurate?

We show the result in both the Gregorian and Hijri calendars (our competitors show only one, or use a rough calculation), we calculate your current week and day precisely, we show your baby's size this week, and we save your result link for sharing. All calculations happen in your browser and never leave your device.

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