Knowing your pregnancy due date is one of the first and most important steps in antenatal care. Your estimated due date (EDD) tells you when your baby is likely to arrive and helps your doctor schedule scans, check-ups, and delivery planning. Whether you are using your last menstrual period, the date of conception, or an IVF transfer record, calculating your pregnancy due date accurately matters from day one.

This guide explains each method in plain language, walks through real examples, and shows you exactly what your EDD means for your pregnancy journey — from the first trimester through delivery.

What Is an Estimated Due Date (EDD)?

An Estimated Due Date — also called Expected Date of Delivery (EDD) or Expected Date of Confinement (EDC) in older medical texts — is the date on which a full-term pregnancy is expected to end. A standard human pregnancy lasts approximately 40 weeks (280 days) from the first day of the last menstrual period (LMP).

It is important to understand that only about 4–5% of babies are born on their exact due date. Research published in obstetric literature consistently shows that deliveries within two weeks either side of the EDD (i.e., between weeks 38 and 42) are considered normal. The EDD is a planning reference, not a guaranteed date.

Method 1: Using Your Last Menstrual Period (LMP) — Naegele's Rule

The LMP method is the most widely used approach in clinical practice and the one most doctors in India use at your first antenatal visit. It is based on Naegele's Rule, developed in the early 19th century by German obstetrician Franz Karl Naegele.

The standard formula is straightforward:

• Take the first day of your last menstrual period.
• Add 1 year, subtract 3 months, and add 7 days.
• Alternatively: add 280 days directly to the LMP date.

This formula assumes a standard 28-day cycle with ovulation on Day 14. If your cycle is longer or shorter, an adjustment is needed:

Method 2: Using Your Conception or Ovulation Date

If you have been tracking ovulation — using basal body temperature, LH surge tests, or fertility apps — you may know your approximate conception date. This method removes the cycle-length variable and can be more precise for women with irregular periods.

The formula uses 266 days (38 weeks) from the conception date, because fertilisation occurs approximately 14 days after the LMP in a standard cycle:

• EDD = Conception Date + 266 days
• Pregnancy is dated as 40 weeks from LMP but only 38 weeks from conception.

Note that even with a known conception date, implantation and early development variations mean the EDD remains an estimate. Ultrasound at 8–12 weeks remains the most accurate method of confirming gestational age.

Method 3: IVF Embryo Transfer — Day 3 vs Day 5

For pregnancies achieved through In Vitro Fertilisation (IVF), the embryo transfer date is known precisely, making the due date calculation highly reliable. The formula depends on the age of the embryo at the time of transfer:

These figures account for the embryo's age relative to fertilisation (Day 0). The total gestational period from fertilisation remains 266 days in both cases.

IVF pregnancies are typically monitored more closely, with early transvaginal ultrasounds at 6–7 weeks to confirm cardiac activity and gestational age.

Understanding the Three Trimesters of Pregnancy

Once you have your EDD, you can map your pregnancy across three trimesters. Each trimester represents a distinct phase of fetal development and comes with specific antenatal milestones.

In India, the Ministry of Health recommends a minimum of 4 antenatal visits during a normal pregnancy, with additional visits for high-risk pregnancies. Your EDD anchors the scheduling of these visits.

How Accurate Is an EDD and When Does It Change?

EDD accuracy depends on the method used and how early the first ultrasound is performed. A first-trimester ultrasound (before 13 weeks) is considered the gold standard for dating a pregnancy, with an accuracy of ±5–7 days. After the first trimester, accuracy decreases.

Your EDD may be revised if:

• The first ultrasound shows a significant size discrepancy from the LMP-based EDD (typically more than 7 days in the first trimester or more than 14 days in the second).
• You have an irregular cycle and the LMP method was not adjusted correctly.
• Your gynaecologist identifies growth restriction or acceleration on later scans.

A revised EDD from ultrasound is generally considered more accurate than the LMP calculation. It is common for doctors at hospitals like Fortis, Narayana Health, or Apollo in India to update EDD after the dating scan.

What Happens After Your Due Date — Post-Term Pregnancy

If your pregnancy extends beyond 40 weeks, it is termed post-term or prolonged pregnancy. Doctors typically take a watchful approach between 40 and 41 weeks. After 41 weeks, closer monitoring with biophysical profiles and non-stress tests begins. Most guidelines recommend considering induction of labour between 41 and 42 weeks to reduce risks of placental insufficiency and stillbirth.

In India, decisions about induction are guided by FOGSI (Federation of Obstetric and Gynaecological Societies of India) recommendations and individual clinical assessment. It is important to stay in close contact with your obstetrician in the final weeks of pregnancy.

Tips for Tracking Your Pregnancy Week by Week

• Keep a record of your LMP: Note it immediately on a calendar or period-tracking app when your cycle ends.
• Track your cycle length over 3 months: If you have a variable cycle, average the lengths for a more accurate LMP-based EDD.
• Schedule a dating scan early: A 7–9 week transvaginal scan or an 11–13 week nuchal translucency scan will confirm your EDD more precisely than any formula.
• Use a pregnancy app or our due date calculator: Digital tools help you monitor your current week, trimester, and upcoming milestones without manual calculation.
• Attend all antenatal appointments: Even if you feel well, regular check-ups detect complications early and keep both you and your baby safe.