What is a pregnancy calculator?
A pregnancy calculator — also called a due date calculator or pregnancy week calculator — estimates when your baby is likely to be born based on the first day of your last menstrual period (LMP) or a known conception date. It also helps you track how far along you are in weeks and which trimester you're in.
This tool uses the standard 280-day (40-week) method that most healthcare providers use as a starting point. It is an informational estimate, not a medical diagnosis or a substitute for professional prenatal care.
How due dates are estimated
The 40-week (280-day) rule
A typical pregnancy lasts about 40 weeks from the first day of your last menstrual period. That's roughly 280 days. This is the foundation of the most common due date formula, often called Naegele's rule.
LMP method
With the LMP method, you take the first day of your last period and add 280 days. This works because pregnancy is counted from the start of your cycle, even though conception usually happens about two weeks later. It's the method most clinicians use when the exact conception date isn't known.
Conception date method
If you know the exact date of conception — for example, from IVF or a known ovulation date — you can estimate the due date by adding 266 days (38 weeks). This is often more precise, but it requires accurate information. Many people don't know the exact conception date, which is why the LMP method is more commonly used.
Trimester timeline
- First trimester (weeks 1–13): Early development, major organ formation, and often the most symptom-heavy period.
- Second trimester (weeks 14–27): Often called the "honeymoon" phase — many people feel more energy and begin to show.
- Third trimester (weeks 28–40+): Final growth, preparation for birth, and more frequent prenatal visits.
Why actual delivery dates vary
Only about 4% of babies are born on their exact estimated due date. Many factors affect timing: cycle length, ovulation timing, first-time pregnancy, multiples, and individual health. Your provider may adjust your due date after an ultrasound, which is often more accurate than LMP-based estimates.
Think of the due date as a window, not a deadline. Most babies arrive within two weeks before or after the estimated date.