Estimated Conception Date: The approximate day fertilization likely occurred. This is an estimate — pinpointing the exact day is rarely possible without medical confirmation.
Fertile Window: The roughly six-day span (five days before ovulation through one day after) when conception is most likely. Sperm can survive up to five days in the reproductive tract, while the egg is viable for about 12 to 24 hours.
Gestational Age: How far along the pregnancy is today, measured from the estimated conception date or last menstrual period. Healthcare providers typically measure from the LMP, which adds about two weeks to the age from conception.
How This Calculator Works
You enter either a known due date or the first day of your last period (with optional cycle length, defaulting to 28 days). From a due date, conception is estimated by subtracting 266 days. From the last period, ovulation is estimated at half the cycle length after the LMP. The fertile window spans five days before to one day after ovulation. Always confirm estimates with a healthcare provider.
Quick Questions
How accurate is a conception date estimate?
It's approximate. Ovulation timing varies from cycle to cycle, and conception can occur up to five days after intercourse. An ultrasound in the first trimester is generally the most reliable way to date a pregnancy.
What if my cycles are irregular?
If your cycle length varies significantly, the estimate will be less precise. Enter your best average cycle length, or use a due date from an ultrasound for a more accurate calculation.
Why does my doctor count gestational age from my last period, not conception?
Because the last menstrual period date is a concrete, known date for most people, while the conception date is estimated. Counting from LMP adds roughly two weeks compared to counting from conception.
Can I use this to plan conception?
The fertile window estimate can be a rough guide, but for family planning, tracking ovulation with tests, basal body temperature, or cervical mucus monitoring is generally more reliable. Consult a healthcare provider for personalized guidance.