Pregnancy Calculator
Pregnancy and due date calculator. Estimate your due date from last menstrual period or conception date, with week-by-week pregnancy timeline.
Calculation Method
How It Works
- 1
Choose your calculation method
Select 'Last Menstrual Period' if you know the first day of your last period, or 'Conception Date' if you know when conception occurred.
- 2
Enter the date
Enter the relevant date. The calculator will immediately show your estimated due date, gestational age, and trimester.
- 3
Review your pregnancy timeline
See key milestones, a progress indicator, and important dates throughout your pregnancy journey.
How Pregnancy Due Dates Are Calculated
Pregnancy due dates are most commonly estimated using Naegele's rule, a formula developed by German obstetrician Franz Karl Naegele in 1812 and still widely used today. The rule calculates the estimated date of delivery (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a regular 28-day menstrual cycle with ovulation occurring on day 14. For women who know their conception date, the due date is calculated by adding 266 days (38 weeks) from conception. Only about 5% of babies are born on their exact due date — most arrive within a two-week window before or after. Pregnancy is divided into three trimesters: the first (weeks 1–12) covers early development, the second (weeks 13–27) includes major growth milestones, and the third (weeks 28–40) is the final preparation for birth. Key milestones include the anatomy scan around week 20, viability at week 24, full-term status at week 37, and the due date at week 40.
Frequently Asked Questions
How accurate is the due date calculation?
The due date calculated using Naegele's rule is an estimate. Only about 5% of babies are born on their exact due date. Most babies arrive within a two-week window before or after the estimated date. Your healthcare provider may adjust the date based on ultrasound measurements.
What is Naegele's rule?
Naegele's rule is a standard method of calculating the estimated due date by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). It assumes a regular 28-day menstrual cycle with ovulation occurring on day 14.
Should I use LMP or conception date?
Most women use the LMP (last menstrual period) method since the first day of the last period is easier to remember. The conception date method is more accurate if you know the exact date of conception, as it eliminates the assumption of a 28-day cycle.
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