Pregnancy Dating
Click on the calendar or choose a quick preset to set the first day of your last menstrual period (LMP).
No LMP Date Selected
Select the first day of your last menstrual period (LMP) on the calendar above, type the date into the field, or choose a quick preset to view your estimated due date, gestational age, and trimester timeline.
Frequently Asked Questions
How is the due date calculated?
The due date (EDD) is calculated with Naegele’s rule: 280 days, or 40 weeks, from the first day of your last menstrual period (LMP). It assumes a regular 28-day cycle.
How accurate is my due date?
A due date is an estimate. Only about 4–5% of babies are born on their exact due date; most arrive within 2 weeks before or after it.
How many weeks pregnant am I?
Pregnancy weeks are counted from the first day of your last period, not from conception. So at conception you are already about 2 weeks pregnant.
What if my cycle is longer or shorter than 28 days?
This calculator assumes a 28-day cycle. If your cycles are regularly longer or shorter, your due date moves later or earlier by about the difference. An early ultrasound gives a more accurate date.
Can an ultrasound change my due date?
Yes. An early ultrasound, ideally before 14 weeks, is the most accurate way to date a pregnancy. If it differs from the LMP date by more than about a week, doctors usually use the ultrasound date.
When does each trimester start?
The first trimester runs from week 1 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth.
What counts as full term?
Babies born from 37 weeks are term. 39 to 40 weeks is full term, 37–38 weeks is early term, 41 weeks is late term, and 42 weeks or more is post-term.
Clinical note: Standard Naegele-style estimate (LMP + 280 days). Assumes a regular 28-day cycle and ovulation around cycle day 14. Due dates vary naturally, and only ~4% of babies arrive precisely on their estimated due date. Always verify dating and fetal development with your healthcare provider and ultrasound biometric measurements.