How many weeks is a pregnancy?

A full-term pregnancy is 40 weeks (280 days) from the first day of the last menstrual period, or 38 weeks (266 days) from conception. Pregnancy is divided into three trimesters: First (weeks 1-12), Second (weeks 13-26), and Third (weeks 27-40). A baby is considered full-term from week 39.

How accurate is a due date calculator?

A due date calculator provides an estimate. Only about 5% of babies are born on their exact due date. Most babies are born within 2 weeks before or after the estimated due date (between weeks 38 and 42). Ultrasound dating in the first trimester is more accurate than LMP-based calculation, especially for women with irregular cycles.

What are the three trimesters of pregnancy?

First trimester (weeks 1-12): major organ development, morning sickness common. Second trimester (weeks 13-26): baby movements felt, anatomy scan at week 20. Third trimester (weeks 27-40): rapid growth, preparation for birth. Each trimester is approximately 13 weeks long.

When is the earliest a baby can be born safely?

A baby is considered full-term at 39 weeks. Early term is 37-38 weeks, when most babies do well. Before 37 weeks is preterm. Viability (with intensive medical care) begins around 24 weeks, though outcomes improve significantly with each additional week. The ideal time for delivery is 39-40 weeks.

🤰 Pregnancy Calculator

Due Date Calculator

Calculate your estimated due date, current pregnancy week, trimester, and key milestone dates. Based on Naegele's rule — the standard method used by healthcare providers worldwide.

💡 Quick Answer: Due date = first day of last period + 280 days (40 weeks). If your LMP was January 1, your due date is October 8. Only 5% of babies are born on the exact due date.

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First Day of Last Menstrual Period

Cycle Length (default: 28 days)

Adjusts due date for cycles shorter or longer than 28 days.

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📚 Source: Naegele's Rule — the standard obstetric method used by healthcare providers worldwide since 1812.

How Is a Due Date Calculated?

A due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This is the standard method used by doctors and midwives worldwide.

If calculating from conception date, add 266 days (38 weeks), since conception typically occurs about 14 days after the start of the last period. Only about 5% of babies are born on their exact due date — most arrive within a 2-week window before or after. An ultrasound in the first trimester provides the most accurate dating.

What Are the Three Trimesters?

Pregnancy is divided into three trimesters, each approximately 13 weeks long. First trimester (weeks 1-12): major organ formation, morning sickness common. Second trimester (weeks 13-26): baby movements felt, gender visible on ultrasound. Third trimester (weeks 27-40): rapid growth, preparation for birth.

Key milestone weeks: Week 8 (heartbeat detectable), Week 12 (end of first trimester, risk of miscarriage drops significantly), Week 20 (anatomy scan, halfway point), Week 24 (viability milestone), Week 37 (early term), Week 39 (full term). This calculator shows all these milestones with their dates personalized to your pregnancy.

How Accurate Is a Due Date?

Only about 5% of babies arrive on their exact due date. Most are born within 2 weeks before or after the estimated date (between weeks 38 and 42).

Factors that affect accuracy: cycle length (the standard calculation assumes a 28-day cycle), ovulation timing, first-trimester ultrasound measurements, and individual variation. Women with longer cycles may have later due dates. Your healthcare provider may adjust the due date based on ultrasound measurements, especially if they differ significantly from the LMP-based estimate.

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Medical Disclaimer: This calculator provides an estimate only and does not constitute medical advice. Due dates are approximate — only about 5% of babies are born on the exact estimated date. Always consult your healthcare provider (OB-GYN, midwife) for accurate pregnancy dating, especially through first-trimester ultrasound. This tool should not replace professional prenatal care.

Pregnancy Due Date Guide: Understanding How Your Due Date Is Calculated

Your estimated due date (EDD) is one of the most important dates in your pregnancy journey. This calculator uses Naegele's Rule — the standard obstetric method used by healthcare providers worldwide since 1812. Understanding how it works and what it means helps you plan for your baby's arrival with realistic expectations.

How Naegele's Rule Works

The formula is straightforward: take the first day of your last menstrual period (LMP), add 280 days (40 weeks), and that is your estimated due date. This assumes a 28-day menstrual cycle and that ovulation occurred on day 14. If your cycle is longer than 28 days, your due date may be later; if shorter, it may be earlier. This calculator adjusts for cycle lengths between 22-35 days for greater accuracy.

How Accurate Is the Due Date?

Only about 5% of babies are born on their exact due date. However, approximately 80% of babies arrive within a 2-week window around the due date (between 38-42 weeks). A pregnancy is considered "full term" at 39-40 weeks, "early term" at 37-38 weeks, and "late term" at 41-42 weeks. Your healthcare provider may adjust your due date based on ultrasound measurements, especially if the first ultrasound suggests a different gestational age than your LMP calculation.

When NOT to Rely on This Calculator

This calculator is for informational purposes and should not replace prenatal care. See your healthcare provider for an accurate dating if: you are unsure of your last period date, you have irregular cycles, you conceived through IVF (which uses a different calculation), or if your ultrasound measurements differ significantly from the LMP-based estimate. Always attend all scheduled prenatal appointments regardless of what any online calculator says.