Pregnancy Calculator
Estimate your due date from your last menstrual period and see your current week of pregnancy, trimester and days along.
Pregnancy Calculator — input your values
What does this health calculator do?
This pregnancy calculator works out your estimated due date by adding 280 days (40 weeks) to the first day of your last menstrual period, the standard clinical estimate. It also shows your current week of pregnancy, the exact number of days you've been pregnant and which trimester you're in.
Because it compares today's date with your LMP, the result updates automatically as your pregnancy progresses. A trimester overview table summarizes what happens in each stage.
The calculation is an estimate, not a medical assessment. Always follow the advice of your obstetrician or midwife, and note that due dates are commonly adjusted after an early ultrasound.
Formula and variables
Gestational age is counted from the LMP, so 'week 1' includes the two weeks before conception.
| Symbol | Meaning |
|---|---|
| LMP | First day of the last menstrual period |
| Days pregnant | Today − LMP, in days |
Worked example
A last menstrual period starting on April 15, 2026, with today being August 1, 2026.
- Days pregnant: April 15 to August 1 = 108 days.
- 108 ÷ 7 = 15 completed weeks and 3 days — week 16.
- Due date: April 15 + 280 days = January 20, 2027.
Answer: Due date January 20, 2027 — currently week 16, 2nd trimester.
Tips and common mistakes
Tips
- Confirm your dates with an early ultrasound, which measures the embryo and can refine the gestational age.
- The trimester dates are approximate — providers may define boundaries a week or two differently.
- Pregnancies normally range from 37 to 42 weeks; a due date is a midpoint estimate, not a deadline.
- Keep track of the first day of your period, because it is the reference point for most pregnancy dating.
Common mistakes to avoid
- Entering the date your period ended instead of the first day it started.
- Assuming the baby must arrive on the due date — full term spans weeks 37 through 42.
- Using the conception date as the start of the pregnancy — gestational age counts from the LMP, roughly two weeks earlier.
Pregnancy Calculator — frequently asked questions
How is my due date calculated?
The standard method adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14.
How many weeks pregnant am I?
Count the days since your last period and divide by 7. The calculator does this for you and shows both the completed weeks and your current week number.
What trimester am I in?
The first trimester covers weeks 1 to 13, the second weeks 14 to 27, and the third weeks 28 to 40. Boundaries vary slightly by source.
Is the due date accurate?
Only about 4% of babies arrive on their exact due date. A normal pregnancy lasts anywhere from 37 to 42 weeks, so treat the date as a guide.
Why does the due date change after an ultrasound?
Early ultrasound measurements of the embryo are often more precise than LMP dating, especially with irregular cycles, so providers may adjust the date.
What if my cycle is irregular?
LMP dating assumes a 28-day cycle. With irregular cycles, an early ultrasound usually provides a more reliable estimate of gestational age.