Pregnancy Conception Calculator

Disclaimer: This calculator is for informational and estimation purposes only. For medical advice or confirmation of accuracy, please consult a certified doctor or gynecologist.

Pregnancy Conception Calculator: Estimate Your Conception Date and Fertile Window

Curious about when conception likely happened? This calculator estimates your conception date and the most likely conception window using whichever date you already know — your due date, the first day of your last period, an ultrasound date, or your IVF embryo transfer date. It also shows your current gestational age based on the estimate.

How it works

Pregnancy dating is calculated from the first day of the last menstrual period (LMP), not the day of conception itself — which is why you're considered "2 weeks pregnant" before conception typically occurs. Working backward from a known due date, ultrasound measurement, or IVF transfer date, the calculator estimates:

• Conception date: typically around day 14 of a 28-day cycle (adjusted automatically if you enter a different cycle length)
• Conception window: a 5-6 day range, since sperm can survive in the body for up to 5 days and the egg is viable for about 24 hours after ovulation
• Due date: 280 days (40 weeks) from LMP, or adjusted equivalents for ultrasound/IVF methods

Why this matters (use cases)

• Understanding pregnancy timelines, especially early in pregnancy before a dating ultrasound
• Cross-checking a due date against a different calculation method (e.g. LMP vs. ultrasound)
• IVF patients wanting a precise due date based on a known transfer date
• Answering the common question "when did I actually conceive?" for personal timeline tracking

How to use this calculator properly

1. Select which date you know: Due Date, Last Period, Ultrasound Date, or IVF Transfer Date
2. Enter the relevant date(s) — for LMP, also enter your typical cycle length if it's not 28 days; for ultrasound, enter the gestational age shown on your scan
3. Click Calculate
4. Review your estimated conception date, conception window, due date, and current gestational age

Frequently asked questions

How accurate is a conception date estimate?

It's an estimate, not a precise measurement. Even with a known LMP, ovulation timing varies between cycles and individuals, so the actual conception date could fall anywhere within the calculated window. An early first-trimester ultrasound is generally considered the most accurate single method for dating a pregnancy.

What if my menstrual cycle isn't 28 days?

Enter your actual average cycle length. Ovulation typically occurs about 14 days before the next period starts (not 14 days after the last one), so a longer or shorter cycle shifts the estimated ovulation and therefore conception date accordingly.

Can an ultrasound be more accurate than LMP-based dating?

Yes, particularly in the first trimester. Early ultrasound measurements of fetal size are considered highly reliable for dating, especially for people with irregular cycles or an uncertain last period date. Dating can shift slightly as pregnancy progresses and more measurements become available.

Is this a substitute for a doctor's due date estimate?

No. This tool provides a planning estimate based on standard clinical formulas. Your healthcare provider may adjust your due date based on ultrasound measurements, examination findings, or other clinical factors specific to your pregnancy.

Why does gestational age start counting before conception?

By clinical convention, pregnancy is dated from the first day of the last menstrual period rather than the estimated conception date, since LMP is typically the most reliably known reference point. This means "gestational age" and "fetal age" differ by about two weeks throughout pregnancy.