Pregnancy Conception Calculator

LMP (First day of last period)

If you know your LMP, enter it here.

Due Date (Estimated)

If you know your due date, enter it instead of LMP.

Usual Cycle Length (days)

Defaults to 28 days. Used to refine ovulation/conception estimate when LMP is provided.

As of Date

Leave blank to use today (UTC) for gestational age.

Pregnancy Conception Calculator – Quick Cheat Sheet

Naegele’s Rule: Due Date ≈ LMP + 280 days (40 weeks).
Conception from LMP: ≈ LMP + (cycle length − 14) days (ovulation day), with a small window around that date.
Conception from Due Date: ≈ Due Date − 266 days.
Gestational Age (GA): days between LMP and the “As of” date, expressed in weeks + days.
Cycle Length: defaults to 28; increasing it shifts ovulation/conception later.
Reality Check: Ovulation and implantation vary; ultrasound dating may adjust the due date.

Edge cases & notes

If both LMP and Due Date are entered and conflict, the tool favors LMP + 280 for calculations and notes the mismatch.
“As of” defaults to today (UTC). All parsing is strict YYYY-MM-DD to avoid locale/DST issues.
Conception “window” shown as ±2 days around the estimated ovulation/central date for readability.