calculator
Pregnancy Week Calculator 🤰

Find out exactly how many weeks pregnant you are, your trimester, your due date, and key milestones — all from your last menstrual period.

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weeks pregnant
Progress through pregnancy0%
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Due Date
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Trimester
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Days Remaining
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Days Elapsed
📋 How To Use
1
Enter your LMP date. Select the first day of your last menstrual period. This is the standard medical starting point for calculating pregnancy age.
2
Select your cycle length. Choose your average menstrual cycle length. The default of 28 days is used medically, but adjusting for your actual cycle gives a more accurate due date.
3
Click Calculate. See your current week, trimester, due date, days remaining, and a pregnancy progress bar — all at a glance.
⚙️ How Pregnancy Weeks Are Calculated
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Naegele’s Rule
The standard due date formula adds 280 days (40 weeks) to the first day of the LMP. This is adjusted based on your actual cycle length vs the standard 28 days.
3️⃣
Trimesters
Trimester 1: Weeks 1–13. Trimester 2: Weeks 14–27. Trimester 3: Weeks 28–40. Each trimester brings different development milestones for the baby.
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Medical Disclaimer
This calculator provides an estimate. Only an ultrasound scan and your healthcare provider can give you a confirmed gestational age and due date.
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Cycle Adjustment
If your cycle is longer than 28 days, ovulation happens later, so your due date shifts forward. Shorter cycles shift the due date earlier by the same number of days.
❓ Frequently Asked Questions
Before modern pregnancy tests, doctors used the last menstrual period as a reliable, known date since the exact date of conception is usually unknown. Even today, this convention is maintained for consistency. Pregnancy is counted from the LMP even though conception typically occurs about 2 weeks into the cycle.
A calculated due date is an estimate. Only about 5% of babies are born on their exact due date. Most births occur within 2 weeks before or after the due date. A first-trimester ultrasound (dating scan) is the most accurate method to confirm gestational age and adjust your due date if needed.
Key milestones include: Week 6 (heartbeat detectable), Week 12 (end of first trimester, reduced miscarriage risk), Week 20 (anatomy scan, often gender reveal), Week 24 (viability threshold), Week 28 (third trimester begins), Week 37 (full term begins), and Week 40 (due date).
If your periods are irregular, the LMP-based calculation may be less accurate. In this case, an early ultrasound (ideally before 13 weeks) is the best way to confirm your gestational age. Your doctor will use the baby’s measurements to estimate the due date more precisely than any calculator can.
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