Pregnancy

1. Identify the first day of your last menstrual period (LMP). This is the starting date used by most due date calculations.
2. Enter your LMP date into the calculator's date field (or select 'conception date' if you know the exact date of conception).
3. If using conception date, the calculator adds 266 days (38 weeks from conception) to estimate your EDD.
4. Click 'Calculate' to instantly see your estimated due date, current gestational age in weeks and days, and your trimester breakdown.
5. Note the result as an estimate — actual delivery may occur between 38 and 42 weeks and is confirmed by ultrasound.

EDD = LMP + 280 days

• EDD — Estimated Due Date — the projected date of delivery, approximately 40 weeks after the start of the last menstrual period.
• LMP — Last Menstrual Period — the date of the first day of the mother's most recent menstrual period before conception.
• 280 days — The standard gestational length assumed by Naegele's Rule, equal to 40 weeks or roughly 9 months and 7 days.

EDD = March 15, 2025 + 280 days = December 19, 2025

Shorthand check: March 15 → add 1 year → March 15, 2026 → subtract 3 months → December 15, 2025 → add 7 days → December 22, 2025 (shorthand gives ≈ Dec 22; exact 280-day count gives Dec 19 due to calendar variation — the 280-day count is the precise result)

Result: Estimated Due Date: December 19, 2025

Adding exactly 280 days to an LMP of March 15, 2025 yields an EDD of December 19, 2025. This means the pregnancy is expected to last until mid-to-late December 2025. Keep in mind that only about 5% of babies are born on their exact due date — delivery within two weeks before or after this date is considered normal. Your healthcare provider will confirm the due date via ultrasound, typically in the first trimester.

Understanding Pregnancy Due Dates

What Is Gestational Age?

Gestational age is measured from the first day of the last menstrual period, not from the actual date of conception. This convention exists because ovulation and conception are difficult to pinpoint precisely, while the LMP date is typically known.

Naegele's Rule

Developed by German obstetrician Franz Karl Naegele in the early 19th century, Naegele's Rule remains the most common clinical method. It assumes:

• A 28-day menstrual cycle
• Ovulation on day 14
• Fertilization shortly after ovulation

For women with cycles shorter or longer than 28 days, the EDD may be adjusted accordingly.

The Three Trimesters

| Trimester | Weeks | Key Milestones | |---|---|---| | First | 1–12 | Implantation, organ formation, first heartbeat | | Second | 13–26 | Movement felt, anatomy scan, rapid growth | | Third | 27–40 | Lung maturation, final weight gain, birth preparation |

Ultrasound Dating

First-trimester ultrasound (between 8–13 weeks) is considered more accurate than LMP-based dating, especially for women with irregular cycles. If the ultrasound EDD differs from the LMP EDD by more than 7 days in the first trimester, clinicians typically adjust the due date.

Full-Term Pregnancy

The American College of Obstetricians and Gynecologists (ACOG) defines full-term as 39–40 weeks 6 days. Babies born between 37–38 weeks 6 days are considered early term, and those born at 41 weeks or beyond are late term.

This content is educational and does not constitute medical advice. Always consult a qualified healthcare provider for personal medical guidance.

• **Using conception date as LMP:** Conception occurs roughly 14 days after LMP. If you enter your conception date in an LMP field, your EDD will be approximately 2 weeks too early.
• **Assuming the due date is exact:** The EDD is a statistical estimate. Fewer than 5% of births occur on the exact calculated due date.
• **Ignoring cycle length:** Naegele's Rule assumes a 28-day cycle. Women with significantly shorter or longer cycles may have an EDD that is off by several days without adjustment.
• **Confusing gestational age with fetal age:** Gestational age counts from LMP, while fetal age (embryonic age) counts from conception — they differ by roughly 2 weeks.
• **Not verifying with ultrasound:** LMP-based calculations should always be confirmed by a first-trimester ultrasound for clinical accuracy.

• Methods for Estimating the Due Date – ACOG Committee Opinion No. 700 — American College of Obstetricians and Gynecologists (ACOG)
• Gestational Age Assessment – StatPearls — National Center for Biotechnology Information (NCBI) / StatPearls
• Pregnancy: Due Dates and Gestational Age — NHS (National Health Service, UK)
• Calculating the Estimated Due Date – MedlinePlus — U.S. National Library of Medicine / MedlinePlus

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