Conception date vs last period date: what's the difference?

Bottom lineThe last menstrual period (LMP) is the first day of your last period and the conventional start used to date pregnancy (40 weeks from LMP), while the conception date is when the egg was fertilized around ovulation, about 2 weeks after LMP in a 28-day cycle; pregnancy uses LMP because most people know it, which is why at 4 weeks pregnant conception was only about 2 weeks earlier. Irregular cycles can make LMP dating off, corrected by ultrasound.

These two dates are about 2 weeks apart, and the difference causes a lot of confusion about how pregnancy is counted.

Last menstrual period (LMP)

• The first day of your last period
• The conventional starting point doctors use to date pregnancy and set a due date
• Pregnancy is counted as 40 weeks from LMP

Conception date

• The day the egg was fertilized, around ovulation, typically about 2 weeks after LMP in a 28-day cycle
• The biological start of pregnancy

Why pregnancy uses LMP

Most people know their last period date but not the exact day they conceived, so LMP is the practical standard. It means that at "4 weeks pregnant," conception was only about 2 weeks earlier - you're counted as pregnant from before you conceived.

When the difference matters

• Irregular or long cycles: ovulation (and conception) may be later than day 14, so an LMP-based due date can be off, and a dating ultrasound corrects it
• Known conception date (for example from IVF or tracked ovulation): gives a more precise estimate

What to do

Use the Due Date Calculator for an LMP estimate, or the Conception Date Calculator if you know when you conceived.

Femora tracks both your period dates and ovulation, so you can estimate either way.