LMP vs Conception Date for Pregnancy Dating

The arithmetic difference

When LMP is useful

LMP is widely available and easy to report when cycles are regular and the date is remembered. It becomes less reliable when cycles vary, bleeding is mistaken for a period, hormonal contraception recently changed, or the date is uncertain.

When conception dating is useful

A conception estimate may be useful when ovulation was closely tracked or intercourse occurred in a narrow window. However, sperm can survive for several days and ovulation tools estimate a fertile period rather than proving the exact moment of fertilization.

Why IVF timing is different

IVF records document transfer date and embryo age, creating a more precise starting point than estimating spontaneous conception. Use the clinic’s records rather than changing dates from memory.

What if the two dates disagree?

Check that both dates were entered correctly and that gestational age was not confused with time since conception. Bring cycle history, tracking information, and any ultrasound results to prenatal care. A clinician decides the official date.

Using the calculator correctly

Choose one method in the pregnancy due-date calculator. Do not enter an LMP date in the conception field or subtract two weeks manually; the tool already handles the difference.