Conception Date Calculator
Works backward from how many days remain until a known due date to estimate when conception occurred (and how far along you are). Switch the basis to LMP to account for your cycle length and luteal phase.
Last updated: May 2026
About this calculator
Conception typically occurs about 14 days before the next expected period, or roughly 266 days before the due date (based on ovulation-to-delivery). The formula used depends on your input: if you know the due date, conception date = due date − 280 days; if starting from LMP, conception date = LMP + (cycleLength − lutealPhase) days, where the luteal phase is typically 14 days. The luteal phase is the fixed post-ovulation window before menstruation; the follicular phase (pre-ovulation) varies. Adjusting the luteal phase length lets the calculator account for women whose cycles are long due to a longer follicular phase rather than a longer luteal phase. The fertile window spans roughly 5 days before ovulation through ovulation day itself.
How to use
Suppose your due date is 100 days away and you calculate from the due date. Conception is about 266 days before the due date, so 266 − 100 = 166 days ago, which is roughly 166 / 7 ≈ 23.7 weeks of pregnancy so far. Result: "Estimated conception about 166 days ago (≈ 23.7 weeks of pregnancy so far)". The LMP basis shifts the 266-day offset by your cycle length and luteal phase.
Frequently asked questions
How accurate is a conception date calculator based on a due date?
Conception date estimates carry the same ±2-week uncertainty as the underlying due date. Because the exact moment of fertilization cannot be pinpointed without daily hormone monitoring, the calculator returns a probable window rather than a single day. First-trimester ultrasound dating reduces that uncertainty significantly. For legal or medical purposes, always consult a healthcare provider for an officially documented timeline.
What is the luteal phase and why does it affect the conception date calculation?
The luteal phase is the second half of the menstrual cycle, running from ovulation to the start of the next period. It is relatively constant at around 12–16 days for most women. The follicular phase (day 1 to ovulation) is the variable part — it lengthens in longer cycles. By subtracting the luteal phase from your total cycle length, the calculator finds the day of ovulation, which is the most likely conception day. Changing the luteal phase input lets you fine-tune this estimate if you track basal body temperature or LH surges.
Why might my conception date differ from what my doctor calculated?
Doctors primarily use LMP and ultrasound crown-rump length measurements to assign gestational age, then back-calculate conception. Slight differences arise because ultrasound dating uses standardized fetal growth curves that may not match your baby's individual growth. Additionally, providers sometimes round to the nearest week. If your period was irregular or you conceived via IVF with a known transfer date, your doctor's date will be more reliable than an LMP-based estimate.