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Pregnancy Due Date Calculator

How Is a Pregnancy Due Date Calculated?

A pregnancy due date (also called Estimated Date of Delivery or EDD) is calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP). This method, known as Naegele's rule, has been the standard since the early 1800s. While it assumes a 28-day cycle with ovulation on day 14, modern calculators can adjust for different cycle lengths. It's important to understand that your due date is an estimate — only about 5% of babies are born on their exact due date. Most births occur within a five-week window between 37 and 42 weeks of gestation. Your healthcare provider may adjust your due date based on early ultrasound measurements, which are most accurate in the first trimester.

Understanding the 5 Calculation Methods

The LMP method adds 280 days to your last period start date, adjusting if your cycle isn't 28 days. The Conception method adds 266 days from the known or estimated ovulation date. Ultrasound dating uses measurements from an early scan to estimate gestational age, then calculates backward to determine your EDD — this is considered the most accurate method when done before 14 weeks. IVF calculations are the most precise because the exact embryo age is known: add 263 days for a Day 3 transfer, 261 for Day 5, or 260 for Day 6. The Known Due Date method works in reverse, showing you all milestones and key dates based on an EDD you've already received from your doctor.

Frequently Asked Questions

How accurate is a due date calculator?

Due date calculators provide an estimate — only about 5% of babies are born on their exact due date. However, about 80% of babies arrive within the 37-42 week window. First trimester ultrasound dating is the most accurate method (±5 days), while LMP calculations can be off by 1-2 weeks depending on cycle regularity. IVF due dates are the most precise because the embryo age is known exactly.

What if my cycle is longer or shorter than 28 days?

If your cycle is not 28 days, your ovulation likely doesn't occur on day 14. For a 35-day cycle, ovulation occurs around day 21, making your due date about 7 days later than the standard calculation. Our calculator automatically adjusts when you enable the cycle length toggle. This adjustment is crucial for accuracy — a 35-day cycle would shift your due date by a full week.

Should I use LMP or ultrasound dating?

ACOG recommends using ultrasound dating if the ultrasound EDD differs from LMP by more than 7 days in the first trimester, more than 10 days in the second trimester, or more than 21 days in the third trimester. Early ultrasound (8-13 weeks) is generally more accurate than LMP, especially for women with irregular cycles.

How is IVF due date different from natural conception?

IVF due dates are calculated from the embryo transfer date rather than LMP. Since the exact embryo age is known, IVF calculations are the most precise. A Day 5 blastocyst transfer adds 261 days (266 − 5 days of embryo development). A Day 3 cleavage transfer adds 263 days. Your fertility clinic typically provides this calculation.

Can my due date change during pregnancy?

Yes, your healthcare provider may adjust your due date based on ultrasound measurements, especially if there's a significant discrepancy with LMP dating. This is most common after a first trimester ultrasound. Once established by early ultrasound, the due date typically shouldn't be changed based on later scans, as measurement accuracy decreases as pregnancy progresses.

What are the three trimesters?

The first trimester spans weeks 1-12 (organ formation and highest miscarriage risk), the second trimester covers weeks 13-27 (most comfortable period, baby movement begins), and the third trimester runs from week 28 to delivery (rapid growth, preparation for birth). Each trimester involves different prenatal tests and developmental milestones.

What does 'gestational age' mean vs. 'fetal age'?

Gestational age is counted from the first day of your last menstrual period (LMP), which is about 2 weeks before conception. Fetal age (also called embryonic age) is counted from actual conception. So at '8 weeks pregnant' (gestational), the embryo is actually about 6 weeks old. Medical professionals use gestational age as the standard.

When should I tell my doctor my due date?

Share your LMP date and any fertility tracking data at your first prenatal appointment (typically 8-10 weeks). Your doctor will likely confirm or adjust your due date with an early ultrasound. If you conceived through IVF, bring your transfer date and embryo day information. Having accurate dating from early pregnancy is important for monitoring fetal growth.