Pregnancy Due Date: How EDD Is Calculated & What to Expect

Finding out you are pregnant is one of the most significant moments in a person's life. Within seconds, one question takes over everything else: when is my baby due?

The estimated due date (EDD) anchors your entire pregnancy — every antenatal visit, every ultrasound scan, every milestone, and every clinical decision is referenced against it. Yet despite its importance, most people don't know how it is actually calculated, why it might change after an ultrasound, or why a "full-term" pregnancy can span a range of five weeks.

This guide explains exactly how your due date is worked out, what each trimester involves, what scans and tests you need and when, and what government support is available to pregnant women in India.

How Long Is a Pregnancy?

A normal pregnancy lasts approximately 40 weeks — counted from the first day of the last menstrual period (LMP). This is sometimes expressed as 280 days or 10 lunar months.

However, conception typically occurs around Day 14 of a standard 28-day cycle — about 2 weeks after the LMP. So from the moment of actual fertilisation, a pregnancy is only about 38 weeks (266 days). The convention of counting from LMP simply makes it easier to standardise dating, since most women know their last period date but not their exact ovulation date.

A pregnancy is considered term if it reaches 37 completed weeks. Delivery between 37 and 42 weeks is considered normal. Before 37 weeks is preterm; after 42 weeks is post-term.

The Three Trimesters

How Is the Due Date Calculated?

There are four main methods of calculating the estimated due date (EDD), depending on what information is available:

Naegele's Rule — The Standard LMP Formula

The most widely used method for estimating EDD from LMP is Naegele's Rule, developed by German obstetrician Franz Karl Naegele in 1812 and still in use worldwide:

Example: If your LMP was 1 January, your EDD by Naegele's Rule is 8 October (add 9 months = 1 October, then add 7 days = 8 October).

Why Your Due Date May Change After an Ultrasound

An early ultrasound (7–13 weeks) measures the crown-rump length (CRL) — the length of the fetus from the top of the head to the bottom of the spine. This measurement is highly accurate for dating and can differ from the LMP-based EDD.

Standard obstetric practice:

• If the ultrasound EDD differs from LMP EDD by more than 7 days in the first trimester, the ultrasound date is used
• If the difference is 7 days or less, the LMP-based EDD is generally kept
• After 20 weeks, ultrasound dating becomes less accurate and is not routinely used to change the EDD

The Indian Antenatal Scan Schedule

India's scan schedule follows FOGSI (Federation of Obstetric and Gynaecological Societies of India) guidelines. Here are the key scans and tests every pregnant woman should know about:

Gestational Diabetes — A Special Concern for Indian Women

Gestational diabetes mellitus (GDM) is far more common in India than the global average. The prevalence of GDM in Indian women is approximately 16–17% — compared to the global average of 6–9%. South Asian women have a significantly higher genetic predisposition to insulin resistance.

GDM is screened at 24–28 weeks using the DIPSI (Diabetes in Pregnancy Study Group India) protocol: a 75g oral glucose tolerance test (OGTT) with a single 2-hour blood sugar reading — no fasting required, making it practical in any setting. A result ≥ 140 mg/dL at 2 hours confirms GDM.

Uncontrolled GDM increases the risk of macrosomia (large baby), birth complications, neonatal hypoglycaemia, and long-term type 2 diabetes risk in both mother and child. GDM is manageable with dietary changes, exercise, and medication if needed.

Important Nutrients During Pregnancy

First Trimester (Weeks 1–13)

• Folic acid 5 mg/day: Neural tube defect prevention — ideally started before conception and continued until 12 weeks. Available free at government hospitals.
• Iron + Folic Acid (IFA): ICMR recommends 180 mg elemental iron + 500 µg folic acid daily. India has high rates of anaemia in pregnancy — IFA is critical.
• Safe home remedies for nausea: Ginger tea, jeera water, small frequent meals. Avoid raw papaya, pineapple, and large amounts of sesame seeds in early pregnancy.

Second Trimester (Weeks 14–27)

• Calcium 1000–1200 mg/day: Essential for fetal bone development. Sources: milk, curd, paneer, ragi, til. Calcium carbonate supplements are commonly prescribed in India.
• Protein 70–100 g/day: Dal, rajma, chana, eggs, milk, paneer. Most Indian women are significantly protein-deficient during pregnancy — this needs active attention.

Third Trimester (Weeks 28–40)

• Iron absorption tip: Take iron tablets at bedtime with water, not with milk or tea. Vitamin C (amla, lemon juice) improves iron absorption. Dark stools are normal and expected.
• Omega-3 / DHA: Walnuts, flaxseeds, and fatty fish (safe twice per week). Important for fetal brain development. Many obstetricians prescribe DHA supplements from the second trimester.
• Avoid: Alcohol (completely), raw or undercooked meat, unpasteurised milk, excessive Vitamin A (liver, cod liver oil), and more than 200 mg caffeine per day (roughly 2 cups of tea).

Government Schemes for Pregnant Women in India

India has several central government schemes that provide financial support, free care, and institutional delivery incentives to pregnant women. Many women are unaware of these benefits — they are available to eligible women across India.

Signs of Labour — When to Go to Hospital

After 37 weeks, knowing the difference between true labour and false alarms can save unnecessary trips to hospital — and more importantly, ensure you arrive in time when it counts.

True Labour Signs

• Regular contractions: Every 5 minutes, lasting 60 seconds, for 1 hour — the "5-1-1 rule". Contractions start in the back and radiate to the front. They get progressively stronger, longer, and closer together.
• Water breaking (rupture of membranes): A sudden gush or continuous trickle of clear or pale yellow fluid. Go to hospital within 1–2 hours even without contractions — risk of infection rises after 18 hours.
• Bloody show (mucus plug): Passage of blood-tinged mucus. Labour usually begins within 24–48 hours. Not an emergency but alert your doctor.

Kick Counting — Your Most Important Home Monitoring Tool

From 28 weeks, count your baby's movements daily. Aim for 10 movements within 2 hours after a meal when the baby is typically most active. Keep a written kick count diary. Reduced movements can be an early sign of fetal distress — report any reduction to your doctor or go to hospital immediately for CTG monitoring. Do not wait until the next day.

What Is "Post-Dates" — Pregnancy Beyond 40 Weeks?

Only about 5% of babies are born on their exact due date. Most arrive within 2 weeks either side. Going past your due date is very common and does not automatically mean something is wrong.

However, pregnancies extending beyond 42 weeks (post-term) carry increased risks — including reduced placental function, decreased amniotic fluid, and higher rates of meconium aspiration. For this reason, most obstetricians in India offer induction of labour between 41 and 42 weeks, depending on fetal wellbeing assessment. The decision is made with the mother after CTG monitoring, biophysical profile, and amniotic fluid assessment.

This article is for informational purposes based on FOGSI and WHO guidelines. It is not a substitute for professional obstetric care. All clinical decisions regarding pregnancy management, EDD confirmation, and delivery planning must be made by a registered obstetrician or gynaecologist.

Built by an MBBS, AFIH Certified Physician in Punjab, India | RxMedCalc.com