Calculate Your Pregnancy Week
Enter your last menstrual period (LMP), average cycle length, and a date to calculate your current pregnancy week, due date, and trimester.
What is a pregnancy week calculator?
A pregnancy week calculator helps estimate how far along a pregnancy is by using the date of your last menstrual period (LMP). This is the same starting point many clinicians use in early prenatal care. The calculator converts elapsed time into gestational age, usually shown as weeks and days (for example, 18 weeks and 3 days).
It can also estimate your due date, likely conception window, and trimester. While these estimates are useful for planning, they are not a medical diagnosis. Your healthcare provider remains the best source for personalized guidance.
How this calculator works
Core inputs
- LMP date: The first day of your last period before pregnancy.
- Cycle length: A 28-day cycle is common, but many people have shorter or longer cycles.
- As-of date: The date you want to calculate your pregnancy week for (usually today).
Basic formula used
The calculator measures the number of days between your LMP and the selected date. That difference is translated into complete weeks and extra days. Estimated due date is calculated with a standard 280-day pregnancy model, then adjusted based on cycle length.
- Gestational age = As-of date − LMP date
- Estimated due date = LMP + 280 days + (cycle length − 28)
- Estimated conception date ≈ LMP + (cycle length − 14)
Understanding your result
Pregnancy age in weeks and days
Most prenatal milestones are communicated in week/day format. For example, “24 weeks, 5 days” means 24 full weeks have passed and you are 5 days into the next week.
Estimated due date
Your due date is an estimate, not a guarantee. Many healthy pregnancies naturally deliver before or after the exact date. The value is best used as a planning anchor for prenatal appointments, leave planning, and personal preparation.
Trimester breakdown
- First trimester: 0 to 13 weeks, 6 days
- Second trimester: 14 to 27 weeks, 6 days
- Third trimester: 28 weeks and beyond
Important pregnancy milestones by week
- Weeks 4–6: Early confirmation, initial symptoms, and first appointment planning.
- Weeks 8–12: Dating scans may help refine gestational age.
- Weeks 18–22: Anatomy ultrasound is commonly performed.
- Weeks 24–28: Glucose screening often takes place.
- Weeks 36–40: Final stretch with frequent prenatal follow-ups.
Accuracy notes and limitations
This calculator is generally most accurate when cycles are regular and LMP is known. If your cycles are irregular, if ovulation happened earlier/later than expected, or if LMP is uncertain, the estimate can shift. In those cases, early ultrasound dating often provides a better clinical timeline.
Also, fertility treatments, recent hormonal contraception changes, or postpartum cycle irregularity can influence dating. Use this tool for educational planning, and confirm with your obstetrician or midwife for medical decisions.
Frequently asked questions
Is pregnancy counted from conception?
In routine obstetrics, pregnancy age is usually counted from LMP, not conception. That means gestational age is typically about two weeks ahead of embryonic/fetal age.
What if I do not know my exact LMP?
Use your best estimate, then ask your provider about dating ultrasound. Ultrasound can help establish a more precise expected due date, especially early in pregnancy.
Can this calculator replace prenatal care?
No. It is a practical estimate tool only. Always seek professional prenatal care for diagnostics, safety advice, and individualized treatment recommendations.
Final note
Tracking pregnancy week by week can make the journey feel more manageable. Use the calculator to understand where you are now, what is coming next, and how to prepare. Then pair that information with regular medical care so your timeline and health decisions stay grounded in professional guidance.