How accurate is an online pregnancy due date calculator?

An online pregnancy due date calculator is highly accurate when you know your last menstrual period (LMP) date. It uses Naegele's Rule — the same formula used by OB-GYNs worldwide — which adds 280 days (40 weeks) to the first day of your LMP. Only about 5% of babies are born exactly on their due date, but most arrive within 2 weeks either side.

How do I calculate my due date from my last period?

To calculate your due date from your last menstrual period (LMP): Take the first day of your last period, add 7 days, then subtract 3 months, and add 1 year. This is Naegele's Rule. For example, if your LMP was January 1, 2026 → January 8 → October 8 → October 8, 2026 is your estimated due date.

What is the difference between gestational age and fetal age?

Gestational age is counted from the first day of your last menstrual period (LMP) and is what doctors and this calculator use — it's typically 40 weeks. Fetal age (or fertilization age) counts from conception, which occurs about 2 weeks after LMP, so fetal age is approximately 38 weeks at birth. Most medical calculations use gestational age.

When do the three trimesters of pregnancy start and end?

The first trimester runs from Week 1 to Week 12 (months 1–3). The second trimester runs from Week 13 to Week 27 (months 4–6). The third trimester runs from Week 28 to Week 40+ (months 7–9). Each trimester brings distinct developmental milestones for your baby and physical changes for the mother.

What is a full-term pregnancy in weeks?

A full-term pregnancy is 39–40 weeks of gestational age. Early term is 37–38 weeks; late term is 41 weeks; post-term is 42+ weeks. Babies born between 37–42 weeks are generally considered healthy. The American College of Obstetricians and Gynecologists (ACOG) recommends waiting until at least 39 weeks for elective deliveries unless medically necessary.

Can I calculate my due date if I don't know my LMP?

Yes! If you know your approximate conception date, you can enter it directly in our calculator — it adds 266 days (38 weeks) to the conception date. Alternatively, an early ultrasound (before 14 weeks) is the most accurate method for dating a pregnancy when the LMP is uncertain or irregular cycles are present.

How many weeks pregnant am I right now?

To find out how many weeks pregnant you are today, enter the first day of your last menstrual period into this calculator. It will instantly tell you your current gestational age in weeks and days, your current trimester, estimated due date, and days remaining until your due date.

What happens at each trimester of pregnancy?

First trimester (Weeks 1–12): Major organs form; heart begins beating at ~6 weeks; risk of miscarriage is highest. Second trimester (Weeks 13–27): Baby's sex can be determined; movements are felt (quickening); risk decreases. Third trimester (Weeks 28–40): Rapid weight gain; baby positions for birth; lungs mature. Each stage has specific prenatal screenings recommended.

📅 Last updated: April 30, 2026 · Free · No Sign-Up

🤰 Pregnancy Health Tool

Free Pregnancy Due Date Calculator 2026

Enter your Last Menstrual Period (LMP) or conception date for an accurate Estimated Due Date (EDD), trimester timeline, week-by-week milestones, and baby size guide — trusted by expectant parents worldwide.

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📅 Calculate Your Pregnancy Due Date

First Day of Last Menstrual Period (LMP) * The first day you started your last period. This is the most common method used by doctors worldwide.

Average Cycle Length (days) Standard cycle is 28 days. Adjust if yours differs — this affects ovulation and conception timing.

Conception / Ovulation Date * The approximate date of conception or positive ovulation test. Calculator adds 266 days (38 weeks) to this date.

Formula used:
EDD = Conception Date + 266 days
(38 weeks from conception = 40 weeks from LMP)

Ultrasound Scan Date * The date your ultrasound was performed.

Gestational Age at Ultrasound

Enter the gestational age confirmed by your sonographer at the scan.

📅 Estimated Due Date (EDD)

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Weeks Pregnant

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Days Until Due

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Trimester

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You're in Week 12

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📊 Trimester Progress

1st Trimester Weeks 1–12

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2nd Trimester Weeks 13–27

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3rd Trimester Weeks 28–40

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📋 Key Pregnancy Dates

🗓️ Week-by-Week Timeline — Click a Week for Details

🔢 Scientific Calculator

What Is a Pregnancy Due Date Calculator — and How Does It Work?

A pregnancy due date calculator (also called an EDD calculator — Estimated Delivery Date) is a free online tool that predicts when your baby is likely to be born. It uses the same medical formula employed by obstetricians, midwives, and gynecologists in the United States, United Kingdom, Canada, Australia, UAE, and across Europe: Naegele's Rule, developed in the early 19th century and still the global clinical standard in 2026.

Naegele's Rule (LMP Method):
EDD = First Day of LMP + 280 days (40 weeks)

Shortcut Formula:
EDD = LMP date + 7 days − 3 months + 1 year

Conception Method:
EDD = Conception Date + 266 days (38 weeks)

How to Use This Due Date Calculator

Simply enter the first day of your Last Menstrual Period (LMP) in the date field. If you have irregular cycles, you can adjust your average cycle length (the default is 28 days). The calculator instantly shows your EDD, current gestational week, trimester, and important milestone dates for your pregnancy journey.

If you know your conception date (from ovulation tracking or IVF transfer), switch to the "By Conception Date" tab. If your doctor confirmed gestational age via ultrasound, use the "By Ultrasound" method for the most medically accurate result — especially important in the first trimester when measurements are most precise.

What Is Gestational Age — and How Is It Counted?

Gestational age is the most common way to measure how far along a pregnancy is. It's counted from the first day of your last period, even though actual conception happens about 14 days later (during ovulation). At full term, gestational age is 40 weeks — but fetal age (from conception) is only 38 weeks. This 2-week discrepancy can cause confusion, but all standard due date calculators worldwide — including this one — use gestational age as defined by the WHO and ACOG.

Three Trimesters Explained

Pregnancy is divided into three trimesters. The first trimester (Weeks 1–12) is when all major organs begin forming. The heart starts beating around Week 6. This is the most critical period for fetal development and the highest-risk phase for miscarriage. The second trimester (Weeks 13–27) is often called the "honeymoon" of pregnancy — nausea typically fades, the baby's sex can be determined by ultrasound, and you begin to feel movements (quickening) around Week 18–22. The third trimester (Weeks 28–40+) is when the baby gains most of its weight, lungs mature, and the baby moves into the head-down position for birth.

How Accurate Is an Online Due Date Calculator?

Our calculator is as accurate as any doctor's estimate based on LMP alone. However, only about 4–5% of babies are born on their exact due date. Most babies (about 80%) arrive within 2 weeks of the EDD — either early or late. An early ultrasound (before 14 weeks) is the most accurate dating method when LMP is uncertain. At 7–9 weeks, ultrasound dating is accurate to within ±5 days; after 20 weeks, accuracy decreases to ±3 weeks. Always consult your healthcare provider for clinical management decisions.

❓ Frequently Asked Questions

An online pregnancy due date calculator uses the same Naegele's Rule as your OB-GYN — it's clinically accurate when you know your LMP. However, only about 4–5% of babies arrive exactly on the EDD. Most are born within 2 weeks either side. For uncertain LMP dates, an early ultrasound is the gold standard for dating.

Take the first day of your last menstrual period and add 280 days (40 weeks). The shortcut: add 7 days, subtract 3 months, add 1 year. Example — LMP January 1, 2026 → January 8 → October 8 → EDD: October 8, 2026. Our calculator does this instantly and adjusts for your cycle length.

Enter the first day of your last period above — the calculator will instantly tell you today's gestational age in weeks and days. It also shows which trimester you're in, your current milestone, and your baby's approximate size this week.

First trimester: Weeks 1–12 (months 1–3). Second trimester: Weeks 13–27 (months 4–6). Third trimester: Weeks 28–40+ (months 7–9). The second trimester is when most people feel their best. The third trimester ends at birth, typically between 37–42 weeks of gestation.

The American College of Obstetricians and Gynecologists (ACOG) defines: Early Term = 37–38 weeks. Full Term = 39–40 weeks. Late Term = 41 weeks. Post-Term = 42+ weeks. Babies born at 37+ weeks are generally considered viable, but full term (39+ weeks) is optimal for healthy development.

Yes — if you know your conception or ovulation date, use the "By Conception Date" tab. If your doctor performed an ultrasound, use the "By Ultrasound" tab and enter the gestational age confirmed at the scan. Early ultrasound (before 14 weeks) is the most precise dating method when LMP is uncertain.

Gestational age is counted from the first day of your last period (LMP) — it's 40 weeks at birth. Fetal age (fertilization age) counts from conception, which happens ~2 weeks after LMP — it's 38 weeks at birth. All standard medical calculations worldwide use gestational age. This calculator uses gestational age.

First trimester (Weeks 8–13): Blood type, CBC, nuchal translucency ultrasound, NIPT screening, STI tests. Second trimester (Weeks 15–20): Anatomy scan ultrasound, quad screen, glucose challenge test. Third trimester (Weeks 28–40): GBS strep test, kick counts monitoring, cervical checks, Group B Streptococcus test, weekly monitoring near due date. Always follow your healthcare provider's personalized schedule.