Pregnancy Calculator (Due Date & Gestational Age)
Use this calculator to estimate your due date, pregnancy week, trimester, and key milestone dates.
How this pregnancy calculator works
This calculator pregnant tool estimates pregnancy timing using standard obstetric dating. In clinical settings, pregnancy age is usually counted from the first day of your last menstrual period (LMP), not from conception. That means pregnancy is counted as about two weeks along at the time of ovulation in a 28-day cycle.
You can calculate from:
- LMP: Most common method used for early due date estimates.
- Known due date: Useful if your clinician has already confirmed a due date.
What your result means
Estimated due date
The due date is commonly estimated as 280 days (40 weeks) from LMP for a 28-day cycle. If your cycle is shorter or longer, this calculator adjusts the estimate accordingly.
Gestational age (weeks + days)
You’ll see your current pregnancy age as weeks and days. For example, “18 weeks + 3 days” means 18 full weeks have passed since LMP and you are 3 days into week 19.
Trimester
- First trimester: 0 to 13 weeks + 6 days
- Second trimester: 14 to 27 weeks + 6 days
- Third trimester: 28 weeks to birth
Major pregnancy milestones to track
The calculator also provides practical milestone dates to help with planning:
- Estimated conception window
- End of first trimester
- 20-week anatomy scan timeframe
- Approximate full-term start (39 weeks)
These dates are planning checkpoints. Exact scheduling varies based on your prenatal care provider, risk factors, and local practice standards.
Why due dates can change
It is very common for due dates to be refined, especially in early pregnancy. A date may shift if:
- Your cycle length is irregular or unknown
- Ovulation occurred earlier or later than average
- Early ultrasound measurements suggest a different gestational age
- LMP date is uncertain
Most babies are not born exactly on the due date. Birth can naturally happen before or after the estimate.
Best practices for using a pregnancy week calculator
1) Use accurate dates
If possible, use the exact first day of your last menstrual period. A one-day error can shift all milestone estimates.
2) Update after clinician review
If your provider gives you a revised due date based on ultrasound, use that as your primary date for future tracking.
3) Focus on trends, not exact day predictions
Pregnancy progress is best viewed by ranges and windows. Labor timing is naturally variable.
Frequently asked questions
Is this calculator accurate for IVF pregnancies?
IVF dating often uses embryo transfer date and embryo age, which can be more precise than LMP methods. Use your fertility clinic’s guidance for best accuracy.
What if my cycles are irregular?
LMP-based estimates can be less precise with irregular cycles. Early ultrasound may provide a better dating estimate.
Can I use this before I miss a period?
You can estimate dates, but pregnancy confirmation requires testing and clinical follow-up.
Final thoughts
A good calculator pregnant tool can reduce uncertainty and help you plan appointments, work schedules, and personal milestones. Use it as a practical guide, then confirm and personalize your timeline with your OB-GYN or midwife.