Pregnancy Calculator

What Is a Pregnancy Calculator?

A pregnancy calculator estimates the key dates of a pregnancy — most importantly the estimated due date (EDD) — from information you already know. The most common input is the first day of your last menstrual period (LMP). From that date, the calculator estimates how far along the pregnancy is (gestational age in weeks and days), which trimester you are in, roughly when conception likely occurred, and how much of a 40-week pregnancy has elapsed.

These results are estimates, not predictions. They are based on population averages, not on your individual ovulation, fertilization, or implantation timing. A healthcare provider may adjust the due date after an ultrasound or a clinical exam.

How to Calculate Your Pregnancy Due Date

The conventional method uses a pregnancy length of about 280 days (40 weeks) from the first day of your last menstrual period. This assumes a 28-day cycle with ovulation around day 14. If your average cycle is longer or shorter than 28 days, the calculator adjusts the due date by the difference.

Worked example

Suppose your LMP started on January 10, 2026 and your average cycle is 28 days:

• 280 days after January 10, 2026 is October 17, 2026 (estimated due date).
• Cycle adjustment: 28 − 28 = 0, so the due date stays the same.
• Estimated conception: LMP + (28 − 14) = LMP + 14 days = January 24, 2026.

If your cycle were 30 days, the due date would shift by 30 − 28 = 2 days later, to October 19, 2026, and estimated conception would move to LMP + 16 days = January 26, 2026.

How to Use This Pregnancy Calculator

1. Select the tab that matches the dating method you know best: Last Period, Conception, Ultrasound, or IVF Transfer.
2. Enter the required date. You can type it (MM/DD/YYYY) or click the arrow to pick from a calendar.
3. Click Calculate.
4. Review your estimated due date, gestational age, trimester, conception date, and progress.
5. Use Download PDF to save a report, or Print Report to print it.
6. Click Reset to clear the panel and start over.

How Many Weeks Pregnant Am I?

Gestational age is counted from the first day of your last menstrual period, not from conception. It is usually written as weeks + days, for example "12 weeks and 3 days." This is the convention healthcare providers use, which is why a pregnancy is described as lasting about 40 weeks even though conception typically occurs about two weeks after the LMP.

For example, if your LMP was 70 days ago, you are approximately 10 weeks and 0 days pregnant by this convention (70 ÷ 7 = 10 weeks exactly). This is a calendar estimate, not a measurement of fetal development.

Understanding Pregnancy Trimesters

Pregnancy is commonly divided into three trimesters. A widely used convention is:

• First trimester: through 13 weeks and 6 days.
• Second trimester: 14 weeks through 27 weeks and 6 days.
• Third trimester: 28 weeks onward.

Clinical definitions can vary slightly between organizations and providers. Your healthcare professional's framing of your trimester is the one that matters for your care.

How Is the Conception Date Estimated?

When using the LMP method, the estimated conception date is calculated as LMP + (cycle length − 14) days. This assumes ovulation occurs about 14 days before the next expected period. For a 28-day cycle that is LMP + 14 days. For a 30-day cycle it is LMP + 16 days.

When using the Conception tab, the date you enter is treated as the conception date directly. When using IVF, the transfer date is used to estimate when conception occurred (approximately 3 days before a Day-3 transfer, or 5 days before a Day-5 transfer).

In all cases, this is a rough estimate. Actual ovulation, fertilization, and implantation can happen on different days, and sperm can survive in the reproductive tract for several days.

How Accurate Is a Pregnancy Calculator?

LMP-based dating is reasonably accurate for people with regular cycles and a reliably remembered LMP. Accuracy decreases when cycles are irregular, the LMP date is uncertain, or ovulation occurred earlier or later than assumed. First-trimester ultrasound measurements are often more accurate than LMP-based dating, and a healthcare provider may revise the due date accordingly.

Pregnancy Due Date Formula

The formulas used by this calculator are:

Last Menstrual Period (LMP): Estimated Due Date = LMP + 280 days + (cycle length − 28 days) Estimated Conception Date = LMP + (cycle length − 14 days) Conception Date: Estimated Due Date = Conception date + 266 days Ultrasound: Estimated Due Date = Ultrasound date + (280 − gestational age at scan in days) IVF Day-3 Embryo Transfer: Estimated Due Date = Transfer date + 263 days IVF Day-5 Blastocyst Transfer: Estimated Due Date = Transfer date + 261 days Gestational Age = calendar days since LMP-equivalent, shown as weeks + days Pregnancy Progress (%) = (days since LMP-equivalent ÷ 280) × 100, bounded to 0–100%

All arithmetic uses calendar-day differences, which avoids timezone-related off-by-one errors.

Frequently Asked Questions