How accurate is a due date calculator?

Due date calculators based on LMP are accurate to within 1–2 weeks for most people. However, individual variation in cycle length and ovulation timing can shift the actual due date. First-trimester ultrasound (8–13 weeks) is the most accurate method for confirming gestational age, with accuracy of ±5–7 days. Later ultrasounds are less accurate for dating.

What does gestational age mean?

Gestational age is the number of weeks of pregnancy counted from the first day of your last menstrual period, not from conception. This means that at the time of conception (approximately week 2–3), you are already considered 2–3 weeks pregnant. A full-term pregnancy is 40 weeks gestational age. Doctors and midwives always use gestational age in weeks for clinical monitoring.

What are the three trimesters of pregnancy?

Pregnancy is divided into three trimesters. The first trimester covers weeks 1–12 and involves the most critical organ development. The second trimester spans weeks 13–26 — often the most comfortable period, with the baby's movements becoming noticeable. The third trimester is weeks 27–40, when the baby gains most of its weight and prepares for birth.

What is considered full term?

A pregnancy is considered full term from 39 weeks 0 days to 40 weeks 6 days. Early term is 37–38 weeks. Late term is 41 weeks. Post term is 42 weeks or beyond. Babies born before 37 weeks are considered preterm. The American College of Obstetricians and Gynecologists recommends avoiding elective delivery before 39 weeks when possible.

When should I take a pregnancy test?

Home pregnancy tests detect hCG (human chorionic gonadotrophin), which first appears in urine around 10–14 days after conception. The most reliable time to test is after a missed period (approximately 14 days after ovulation). Testing earlier may give a false negative. A positive result should be followed up with confirmation from a healthcare provider and booking of a first prenatal appointment.

Pregnancy Due Date Calculator

Calculate your estimated due date from your last menstrual period, conception date, or IVF transfer. Track your trimester, pregnancy week, and key milestones.

Calculation Method

First Day of Last Period

How Due Dates Are Calculated

Pregnancy due dates are calculated using **Naegele's Rule**, developed by German obstetrician Franz Karl Naegele in the 1800s. The rule adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This corresponds to the average length of a human pregnancy from the last period to birth.

The 280-day estimate assumes a regular 28-day menstrual cycle with ovulation on day 14. If your cycle is significantly shorter or longer than 28 days, your actual due date may differ — your healthcare provider can give a more personalised estimate, often confirmed by ultrasound.

If you know your conception date, the calculation adds 266 days (38 weeks from conception, since conception typically occurs about 14 days after the LMP).

It is important to note that a due date is an estimate, not a certainty. Only about 5% of babies are born on their exact due date. Most births occur within a two-week window before or after the estimated date. Gestational age is typically confirmed and refined by first-trimester ultrasound.

Frequently Asked Questions

A pregnancy 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 midwives and obstetricians worldwide. If you know your conception date, 266 days are added instead. Due dates are always estimates — ultrasound confirmation is more accurate, especially in the first trimester.

Related Calculators

All tools

⚖️

BMI Calculator

Calculate your Body Mass Index using the WHO standard formula. Get your weight category and healthy weight range instantly.

🔥 Popular

🔥

Calorie Calculator

Calculate your BMR and TDEE using the Mifflin-St Jeor equation. Get personalized daily calorie targets for any goal.

🔥 Popular

⚡

TDEE Calculator

Calculate your Total Daily Energy Expenditure — the cornerstone of any effective diet plan.

🔥 Popular

🥗

Macro Calculator

Get science-based macronutrient targets tailored to your calorie needs and fitness goal.

🔥 Popular