π Conception Date Calculator
Work backward from your due date or last period.
Estimates from standard dating math. Ultrasound dating in early pregnancy is more precise.
Conception happens about two weeks after the last period starts, or 38 weeks before the due date. Working backward from either date gives the conception estimate, and the fertile window stretches about five days before it because sperm survive that long. Enter your due date or last period above for the estimate and its honest precision.
How to Use This Calculator
Pick your starting point: due date or last menstrual period. The conception estimate appears.
Choose due date (from ultrasound or LMP dating) or last menstrual period with your cycle length.
Read the estimated conception date plus the fertile window around it.
Remember the precision: within a few days, not to the hour. Early ultrasound dating beats all math.
The Dating Math
Pregnancy is dated from the last period, not from conception: the famous 40 weeks includes two pre-conception weeks, so conception sits at "2 weeks pregnant" by convention. Subtracting 266 days (38 weeks) from the due date reverses that. From LMP, ovulation lands about 14 days before the next period, not 14 days after the last one, which is why the formula uses cycle length minus 14: in a 35-day cycle, ovulation is around day 21, not day 14.
The fertile window exists because sperm live up to 5 days in fertile cervical mucus while the egg survives about 24 hours. Intercourse any time in the five days before ovulation can produce a pregnancy, so "conception date" really means the ovulation date, with a tail of possible intercourse dates behind it. Paternity questions need DNA, not calendars: the window is several days wide by biology.
Worked Example
Due date October 8, 2026: conception = October 8 minus 266 days = January 15, 2026. Fertile window roughly January 10-15. From LMP January 1 with a 28-day cycle: conception = January 1 + 14 = January 15. Both routes agree, as they should, since the due date itself came from the LMP. With a 35-day cycle and LMP January 1: conception = January 1 + 21 = January 22, a full week later. Cycle length is the variable most people forget, and it moves the answer by days.
Why Ultrasound Beats the Calendar
First-trimester ultrasound (crown-rump length) dates pregnancy within 5-7 days, better than LMP math whenever cycles are irregular, ovulation was early or late, or the LMP is uncertain. Our CRL calculator converts those measurements to gestational age. When ultrasound and LMP disagree by more than the method's error, obstetricians redated to the ultrasound: biology outranks arithmetic. The conception estimate then follows from the redated due date.
Common confusions this calculator resolves: a positive test "3 weeks pregnant" means conception was about a week ago (tests detect hCG after implantation, ~6-12 days post-conception). Bleeding around the expected period can be implantation bleeding, not a true period, which shifts LMP-based math. And gestational age at birth: only about 5 percent of babies arrive on the due date; 37-42 weeks is the full-term range. For the forward direction, our due date calculator runs the same math in reverse.
Precision Limits, Stated Plainly
This estimate is good to a few days under ideal conditions (known LMP, regular cycles, first-trimester dating) and good to a week or more otherwise. It cannot identify which day intercourse led to conception within the fertile window, and it cannot resolve paternity. It also assumes ovulation happened; anovulatory cycles produce no conception at all. Treat the date as the center of a window, not a timestamp, and let the ultrasound have the last word.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
How do I calculate my conception date?
From your due date: subtract 266 days (38 weeks). From your last period: add (cycle length - 14) days. Example: LMP January 1 with a 28-day cycle gives January 15. This calculator does both.
How accurate is a conception date calculator?
Within a few days under ideal conditions (regular cycles, certain LMP). First-trimester ultrasound dating is more precise. The fertile window spans about 5 days before ovulation, so exact-day precision is biologically impossible from dates alone.
Can conception date determine paternity?
No. The fertile window covers several days, and sperm survive up to 5 days, so date math cannot identify which act of intercourse caused pregnancy. Only DNA testing determines paternity.
Why is pregnancy dated from the last period instead of conception?
Convention: the 40-week gestational age includes ~2 weeks before conception, because LMP is an observable date while conception is not. So at conception you are already "2 weeks pregnant" by dating convention.
What if my cycles are irregular?
Then LMP-based conception dating is unreliable, since ovulation does not sit 14 days after LMP. Use the due date from first-trimester ultrasound instead; it measures the embryo directly and does not depend on cycle regularity.
How soon after conception can I test positive?
Home tests detect hCG after implantation, about 6-12 days post-conception, so roughly 3-4 weeks gestational age (1-2 weeks after the missed period). Testing earlier risks false negatives.
Does the conception date change with IVF?
IVF dating is exact: gestational age is calculated from egg retrieval (minus 14 days) or embryo transfer date plus embryo age. IVF pregnancies do not need conception estimation.
What is Naegele's rule?
Due date = LMP + 1 year - 3 months + 7 days (for 28-day cycles). This calculator reverses related math: conception sits 266 days before that due date. Naegele published it in the 1800s; ultrasound has refined but not replaced it.
Can implantation bleeding be mistaken for a period?
Yes, and it shifts LMP-based conception math by a full cycle if the "period" was actually implantation bleeding. If dates and ultrasound disagree, trust the ultrasound and mention the bleeding to your provider.
How long is the fertile window?
About 6 days: the 5 days before ovulation plus ovulation day, set by 5-day sperm survival and ~24-hour egg survival. The 2-3 days before ovulation are the most fertile.