Pregnancy Conception Calculator

Conception typically occurs around the time of ovulation, which is usually 11–21 days after the first day of your last menstrual period. For a standard 28-day cycle, ovulation — and therefore conception — happens roughly 14 days after your LMP. This calculator estimates that date based on whichever input method you choose.

The estimated conception date is just that — an estimate. Conception dates can vary based on cycle length, ovulation timing, and sperm viability (sperm can survive 3–5 days inside the body). Ultrasound dating in the first trimester is generally the most accurate method, with a margin of about ±5–7 days.

Once the conception date is estimated, 266 days (38 weeks) are added to arrive at the estimated due date. Alternatively, using the LMP method, 280 days (40 weeks) are added to the first day of the last menstrual period, which accounts for the roughly 14-day gap before ovulation.

The conception window is a range of dates during which sexual intercourse could have led to conception. It accounts for the fact that sperm can survive for 3–5 days inside the female reproductive tract, meaning intercourse in the days leading up to ovulation can still result in fertilization.

Yes. Your doctor may adjust your due date based on ultrasound measurements, particularly if you have irregular cycles or are unsure of your LMP. First-trimester ultrasounds are the most accurate for dating a pregnancy and may shift your estimated due date by a week or more.

Gestational age is most commonly counted from the first day of your last menstrual period (LMP), even though conception actually occurs about two weeks later. Your healthcare provider may also use ultrasound measurements to confirm how many weeks and days pregnant you are.

Yes. Women with longer cycles ovulate later, which shifts the estimated conception date forward. Women with shorter cycles ovulate earlier. This calculator adjusts the ovulation estimate by comparing your cycle length to the standard 28-day cycle, adding or subtracting days accordingly.

If you already know your due date — as given by your doctor or a previous ultrasound — simply select 'Due Date' as your calculation method and enter it. The calculator will work backward 266 days to estimate your conception date and display your possible conception window.