Conception Calculator – Pinpoint the Date
Work backward from your due date to discover your likely date of conception. Understand your fertile window and the biology behind the timeline.
Due Date
Likely Conception Window
Many expectant parents are confused when they hear they are "4 weeks pregnant" but only conceived 2 weeks ago. This discrepancy exists because medical pregnancy dating typically counts from the Last Menstrual Period (LMP), not the actual biological event of fertilization.
This calculator clarifies that confusion. By entering your due date (usually established by ultrasound), we subtract exactly 38 weeks (266 days) to reveal the approximate day ovulation and fertilization occurred.
Biological Age vs. Gestational Age
Your baby's Gestational Age (used by doctors) is always roughly 2 weeks older than their Fetal Age (actual age from conception). This calculator solves for Fetal Age.
The 5-Day Window
Finding an exact "Conception Day" is tricky because sperm can wait for the egg. Conception technically happens at ovulation, but the intercourse that led to it could have happened up to 5 days prior.
The Science of Fertilization
Conception is a complex biological relay race. Here are the key phases that determine the date.
Why Ultrasound Dates Win
If your ultrasound due date differs from your period-based due date, trust the ultrasound.
Early embryos grow at a very predictable rate. Measuring the "Crown-Rump Length" (CRL) in the first trimester allows doctors to date the pregnancy within 3-5 days of accuracy, bypassing any errors caused by irregular ovulation.
Planning Future Pregnancies
Tracking for Accuracy
If you want to know your conception date precisely for a future pregnancy, rely on data, not just the calendar.
- OPKs (Ovulation Predictor Kits):These detect the LH surge that happens 24-36 hours before ovulation.
- BBT (Basal Body Temperature):Your resting temperature spikes slightly after ovulation, confirming that it occurred.
Reverse Engineering the Date
Medical due dates are typically set 40 weeks (280 days) from the last period. To find the conception date, we work backward from the Estimated Due Date (EDD).
We also provide a Conception Window (typically ±5 days) because sperm can survive in the body for up to 5 days before ovulation, meaning intercourse on Tuesday could result in conception on Saturday.
Timeline Examples
Standard Delivery
Due Date: December 25th
Conception Math: Dec 25 - 38 weeks
Conception Date: April 3rd
Likely intercourse window: March 29th - April 4th.
Preterm Birth
Actual Birth: Nov 25th (36 weeks)
Original Due Date: Dec 25th
Conception Date: Still April 3rd
Birth date does not change conception date. Use the Due Date (40 week mark) for calculation.
Related Tools
How accurate is this conception date?
It is an estimate rooted in averages. It assumes a standard 266-day gestation from conception. If you know your ovulation date confirmed by ultrasound, that is more accurate.
Can DNA testing determine conception date?
No. Paternity tests can identify the father, but they cannot pinpoint the exact hour or day conception occurred.
I had sex 3 days before this date. Is that possible?
Yes! Sperm can live inside the female reproductive tract for up to 5 days waiting for an egg. You likely conceived on the date shown, resulting from intercourse days prior.
Does IVF have a different calculation?
Yes. For IVF, the conception date is known exactly (the day of fertilization in the lab). Due dates are calculated by adding 266 days to the egg retrieval/fertilization date.
Why do doctors add 2 weeks to my pregnancy?
Doctors count from the Last Menstrual Period (LMP) because it is the only visible date a patient knows. Conception is invisible. So 'Day 1' of pregnancy is actually the first day of your period, 2 weeks before you are even pregnant.
Fertility Dictionary
Ovulation
The release of a mature egg from the ovary. It is the only time during a cycle that conception is possible.
Gestation
The period of development from conception to birth. In humans, full term is roughly 38 weeks (fetal age) or 40 weeks (gestational age).
Zygote
The single cell formed by the union of sperm and egg. It immediately begins dividing and becomes a blastocyst.
Implantation Bleeding
Light spotting that can occur 6-12 days after conception when the egg attaches to the uterus. Often mistaken for a light period.
About This Calculator
We utilize standard obstetric algorithms that assume a 280-day gestational period. This is aligned with the protocols used by sonographers and the American Institute of Ultrasound in Medicine (AIUM) for dating pregnancies.