Calculate Pregnancy Week, Due Date, and Trimester
Use this pregnancy calculator to estimate your current gestational age based on either your last menstrual period (LMP) or your estimated due date.
How this gestational week calculator works
Gestational age is usually measured from the first day of your last menstrual period (LMP). This is the standard approach used in obstetrics and is why pregnancy is commonly listed as 40 weeks, even though conception usually happens around 2 weeks after LMP in a 28-day cycle.
This calculator gives you an estimate of:
- Your current gestational age in weeks and days
- Your current pregnancy week number
- Your estimated due date
- Your likely trimester
- Key milestone dates across pregnancy
Method 1: Last menstrual period (LMP)
If you know the first day of your last period, this method is typically the best starting point. The tool also allows cycle-length adjustment. If your cycle tends to be longer or shorter than 28 days, your estimated due date can shift a little.
Method 2: Estimated due date
If your clinician has already provided a due date (often based on ultrasound), choose this method. The calculator will work backward to estimate gestational age and related milestones.
How to use the calculator correctly
- Select your calculation method.
- Enter your date (LMP or due date).
- If using LMP, enter your average cycle length.
- Choose the “as of” date (today is pre-filled).
- Click Calculate to see your results.
For the most accurate dating, especially if cycle timing is irregular, combine this estimate with professional prenatal care and ultrasound guidance.
Understanding your result
Gestational age
Displayed as X weeks, Y days. For example, 21 weeks and 3 days means 21 full weeks have passed since gestational dating began, and you are currently in week 22.
Trimester ranges
- First trimester: 0 to 13 weeks
- Second trimester: 14 to 27 weeks
- Third trimester: 28 weeks to birth
Milestones you may see
- 12 weeks: end of early first-trimester phase for many people
- 20 weeks: common timing for anatomy scan
- 24 weeks: commonly discussed threshold of viability with intensive care
- 37 weeks: early term begins
- 40 weeks: estimated due date
Common questions
Is gestational age the same as fetal age?
No. Gestational age starts at LMP. Fetal age (time since conception) is usually about two weeks less than gestational age in a 28-day cycle.
Why can due dates change?
Due dates are estimates, not guarantees. Cycle variation, ovulation timing, implantation timing, and updated ultrasound measurements can all shift the expected date.
How accurate is an online pregnancy week calculator?
It is useful for planning and education, but it does not replace clinical assessment. Your healthcare professional may refine dating based on ultrasound and your complete medical history.
Important note
This calculator is for informational use only and is not medical advice. If you have pain, bleeding, reduced fetal movement, or concerns about pregnancy timing, contact your OB-GYN, midwife, or local emergency services promptly.