Conception Date Calculator

How Conception Date Is Calculated

Standard pregnancy duration is 40 weeks (280 days) from the last menstrual period (LMP), not from conception.
Conception typically occurs about 2 weeks after the LMP during ovulation.
So conception date ≈ Due date − 38 weeks (266 days).
Obstetricians use the LMP as the reference point because it is easier to track than the actual fertilization date.

Why There Is a Range, and Why It Is Lopsided

Sperm can survive in the reproductive tract for 3 to 5 days after intercourse.
The egg survives only 12 to 24 hours after ovulation.
Ovulation itself varies by 2 to 3 days from cycle to cycle.

Put those together and the fertile window is not symmetrical around ovulation. It runs from five days before ovulation to the day itself, because sperm can wait for the egg but the egg cannot wait for sperm. Wilcox and colleagues (1995), following 221 women, traced no pregnancies to intercourse after ovulation day. The window this calculator reports is shaped that way, and it is the same six days the site’s ovulation calculator shows. If you are trying to work out which encounter led to a pregnancy, the days before the estimated ovulation date are the likely ones, and the calendar cannot narrow it further than that.

Cycle length changes everything, and in the direction people expect least

Ovulation happens about 14 days before the next period, not 14 days after the last one. So on a 35-day cycle you ovulate around day 21, not day 14, and both conception and the due date sit a week later than the standard formula assumes. On a 21-day cycle they sit a week earlier. Get this backwards and the due date lands two weeks out, which is the difference between “term” and “preterm” on paper.

Whatever the cycle length, the interval from conception to the due date is always 266 days. That is the check worth doing on any calculator of this kind.

Naegele’s Rule (Due Date Formula)

The standard formula for estimating due date from LMP:
Due Date = LMP + 9 months + 7 days (or equivalently, LMP + 280 days).
Named after Franz Karl Naegele, a German obstetrician of the early 1800s.
This assumes a regular 28-day cycle. Adjustments are made for longer or shorter cycles.
Ultrasound dating in the first trimester (8–13 weeks) is more accurate than LMP dating and is used when there is a discrepancy.

First Trimester Development Timeline

Week 1–2: The pre-embryonic period. No conception yet; the body is preparing for ovulation.
Week 3: Fertilization and implantation. The fertilized egg becomes a blastocyst.
Week 4: The embryo implants in the uterine wall. hCG hormone begins rising, so home tests can detect pregnancy.
Week 5–6: The heart begins beating. The neural tube (brain and spinal cord) forms.
Week 8: All major organs are forming.
Week 10: The embryo is now called a fetus (the fetal period starts 10 weeks after the LMP, 8 weeks after conception). Fingers, toes, and facial features are visible on ultrasound.
Week 12: End of first trimester. Miscarriage risk drops significantly. Nuchal translucency scan.
Week 20: Mid-pregnancy anatomy scan. Sex determination is usually possible.

Accuracy of Conception Date Estimates

LMP-based estimates have a margin of error of ±2 weeks.
Ultrasound dating before 14 weeks is accurate to about ±5–7 days, according to the American College of Obstetricians and Gynecologists (ACOG).
At 16 to 22 weeks the margin is about ±10 days, and from 22 to 28 weeks about ±14 days.
DNA paternity testing can confirm biological parenthood but cannot narrow conception to a specific day.
No method can pinpoint an exact conception date. All results are estimates.