Conception Calculator
Pick the starting point you have. We'll estimate your likely conception date and fertile window.
How we estimate conception
This calculator estimates ovulation by using a 14-day back-count from the next expected period, then maps conception to the fertile window around that estimate. The fertile window is a 6-day span that ends on ovulation day.[2] To project forward from there, use our Due Date Calculator. For a 28-day cycle the estimate lands around day 14. We adjust for longer or shorter cycles so the estimate makes sense for you.
For ultrasound dating, we work backward from the gestational age the scan measured.[1] For IVF, the calculator uses the transfer date and embryo age to work back to the fertilization date. After birth, the Baby Percentile Calculator can help put weight and length measurements in context.
Read more: Due Date vs Conception Date: How They Differ.
Frequently asked questions
How is conception date estimated?
For a regular cycle, the calculator uses a 14-day back-count from the next expected period to estimate ovulation. It then maps conception to the fertile window around that estimate.[2] A 28-day cycle puts the estimate around day 14; a 32-day cycle pushes it to around day 18. The calculator uses your cycle length to land on a narrow date range rather than a single exact day.
Can you calculate conception for IVF?
Yes. For IVF, the calculator uses the transfer date and embryo age to work back to the fertilization date. Because IVF timing is observed directly, the resulting conception date is more precise than a natural-cycle estimate and is what your fertility clinic will use for dating.
What is the fertile window?
The fertile window is a six-day span that ends on ovulation day.[2] Because the fertile window includes days before ovulation, conception can result from intercourse before the estimated ovulation day. The two days before and including ovulation carry the highest chance of conception.
Can an ultrasound tell me the exact conception date?
An early ultrasound measures the fetus and estimates gestational age in weeks and days. We work backward from that measurement to infer conception. It is the most accurate non-IVF method, especially in the first trimester.[1] After about 14 weeks, fetal size varies more between healthy pregnancies, so the estimate loses precision.
Is conception the same as implantation?
No. Conception, also called fertilization, is when the sperm and egg join. Implantation happens later, when the early embryo attaches to the uterine lining.[3] This calculator estimates timing; it cannot tell whether implantation happened.
How does cycle length change the conception date?
In cycle-length mode, the calculator uses a 14-day back-count from the next expected period to estimate ovulation, then maps the likely conception window around that estimate. A longer cycle pushes ovulation and conception later in the cycle; a shorter cycle pulls them earlier. The calculator accepts cycle length between 20 and 45 days and applies this shift automatically.