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What is a pregnancy due date?

The due date — also called the estimated date of delivery (EDD) or estimated date of confinement (EDC) — is the projected day a baby is expected to be born. It's based on a 40-week (280-day) gestation counted from the first day of the last menstrual period (LMP). Only about 4% of babies are actually born on their exact due date; most arrive within a 2-week window around it (between 38 and 42 weeks).

The calculator supports three input methods, each with different accuracy and use cases:

• Last menstrual period (LMP) — the most common method, using Naegele's rule. Cycle-length-adjustable.
• Conception date — if you know it precisely (ovulation tracking, single intercourse event), this is more accurate than LMP.
• IVF transfer date — for IVF pregnancies, the conception equivalent is precisely known from the transfer date and embryo age.

How to use the calculator

1. Pick the method that matches what you know.
2. Enter the relevant date.
3. For LMP: also enter your typical cycle length (default 28).
4. For IVF: enter the embryo's age at transfer (typically 3 or 5 days).
5. The due date appears with current gestational age in weeks + days, the trimester (1, 2, or 3), and the days remaining (or past due).

Naegele's rule

Franz Karl Naegele was a 19th-century German obstetrician who codified the formula obstetricians still use:

The 280 days reflect:

• ~14 days from LMP to ovulation (in a textbook 28-day cycle).
• ~266 days of fetal development from conception to full-term birth.
• 14 + 266 = 280 days = 40 weeks.

For people with non-28-day cycles, the calculator adjusts: if your cycle is 32 days (4 days longer), ovulation is 4 days later, so the due date shifts 4 days later. If your cycle is 25 days (3 days shorter), the due date is 3 days earlier.

Why count from LMP and not conception?

Two reasons:

1. Most people know their LMP precisely. The first day of bleeding is unmissable. Conception, on the other hand, often happens days after the act of intercourse (sperm can wait up to 5 days for the egg) — it's hard to nail down without ovulation tracking or IVF.
2. Convention. Obstetric weeks are counted FROM the LMP. So when you're "10 weeks pregnant," that means it's been 10 weeks (70 days) since your LMP, but only about 8 weeks (56 days) of actual fetal development. This sometimes confuses new parents — a "10-week scan" is looking at an 8-week-old fetus.

How to use a specific conception date

If you know exactly when conception happened (single act of intercourse, ovulation predictor kits showing the LH surge, IVF), use:

This is more precise than LMP because it removes the 14-day variability of the follicular phase. For pregnancies conceived through tracked ovulation, the conception-date method is sometimes used by doctors to set the due date.

IVF due date math

IVF pregnancies have the most precise dating possible — both the embryo's age at transfer and the transfer date are known to the day. The formula:

• Day-3 cleavage stage: due date = transfer + 263 days.
• Day-5 blastocyst: due date = transfer + 261 days.
• Day-6 blastocyst: due date = transfer + 260 days.

The "embryo age" is the number of days since fertilization in the lab. So a day-5 transfer of a frozen blastocyst that was thawed and transferred immediately doesn't change the math — the embryo's biological age at transfer is what counts.

The accuracy of due dates

Here's the truth: due dates are estimates with substantial spread.

• ~4% of babies born on the exact due date.
• ~80% born within 2 weeks (38-42 weeks).
• ~30% born exactly on or in the week of the due date.
• ~10% arrive before 37 weeks (preterm).
• ~5% arrive after 42 weeks (post-term — modern care typically induces delivery before this).

First-time mothers tend to deliver slightly later than expected (often 4-7 days after the due date). Subsequent pregnancies are usually closer to the due date or slightly earlier. This is statistical, not deterministic — every pregnancy is its own.

Ultrasound dating

Early ultrasound (especially at 7-10 weeks) is more accurate than calendar methods for setting a due date. The crown-rump length (CRL) of the fetus at this stage is highly correlated with gestational age — accurate to within ~5 days.

If your LMP-based due date and your early-ultrasound due date differ by more than 5-7 days, doctors usually go with the ultrasound. Later ultrasounds (after 20 weeks) become less reliable for dating, since fetal growth varies more by genetics and environment after that point.

The calendar calculator is a useful starting point, but treat it as approximate. Your healthcare provider will likely refine the date with imaging.

The trimesters

The 40-week pregnancy is divided into three trimesters, each with characteristic events:

First trimester (weeks 1-12 or 1-13)

• Implantation, placenta formation, all major organs forming.
• Highest miscarriage risk (most miscarriages happen by week 13).
• Common symptoms: morning sickness, fatigue, breast tenderness, frequent urination, food aversions.
• Heartbeat detectable on ultrasound around week 6.
• NIPT (non-invasive prenatal testing) blood test typically offered around week 10.

Second trimester (weeks 13-26)

• Often called the "honeymoon trimester" — most morning sickness resolves, energy returns.
• Quickening (first felt fetal movement) typically between weeks 16-22.
• Anatomy scan ultrasound around weeks 18-22 — checks anatomy, often reveals biological sex.
• Glucose tolerance test for gestational diabetes around weeks 24-28.

Third trimester (weeks 27-40)

• Rapid fetal growth and weight gain.
• Increasing physical discomfort: back pain, swelling, frequent urination, sleep difficulty.
• Braxton-Hicks contractions become more noticeable.
• Group B strep test typically around 35-37 weeks.
• Position checks — head-down by ~36 weeks for vaginal delivery.
• Term: 37+ weeks. Most providers won't induce before 39 weeks unless medically indicated.

What if you go past 40 weeks?

You're not "late" until 41-42 weeks. Many providers will offer membrane sweeping (manual) or induction (with medication) around 41 weeks if labor hasn't started naturally, since post-term pregnancies (42+ weeks) have slightly elevated risks. Induction policies vary by provider and region.

Going slightly past your due date is normal. It doesn't mean anything is wrong — it just means your baby's individual gestational length is on the longer end of the normal range.

What if you go into labor early?

Preterm = before 37 weeks. About 10% of US births are preterm. Outcomes depend dramatically on how early:

• 34-37 weeks (late preterm): generally good outcomes; may need short NICU stay.
• 32-34 weeks (moderate preterm): NICU stay likely; most do well long-term.
• 28-32 weeks (very preterm): significant NICU support; outcomes have improved dramatically with modern care.
• Under 28 weeks (extremely preterm): challenging, but viability has improved significantly with medical advances.

If you're going to deliver early, you'll be hospitalized and the medical team will guide everything from there.

Common questions and concerns

"Can I trust the due date my doctor gave me?"

If it was set by an early ultrasound (before 13 weeks), yes — that's the most accurate dating method. If based on LMP only and you have irregular cycles, the date is approximate.

"Should I tell people my exact due date?"

Many people give a due date range ("late June") rather than a specific date, because actually delivering on the date is uncommon and the constant "any day now?" texts get tiring as you approach.

"What does 'full term' mean?"

The American College of Obstetricians and Gynecologists defines:

• Early term: 37 0/7 to 38 6/7 weeks
• Full term: 39 0/7 to 40 6/7 weeks
• Late term: 41 0/7 to 41 6/7 weeks
• Post-term: 42 0/7 weeks or longer

Most providers consider 39+ weeks the safest delivery window.

What the calculator gives you, summarized

• Due date — based on the method you picked (LMP, conception, or IVF transfer).
• Current gestational age — in weeks and days, calculated from today's date.
• Trimester — 1, 2, or 3, with boundaries at week 13 and week 27.
• Days to go (or days past) — countdown to (or beyond) the due date.
• Conception estimate — derived from your inputs, useful for context.
• Cycle-length adjustment — Naegele's rule modified for non-28-day cycles in LMP mode.
• Privacy by design — runs entirely in your browser; no data leaves your device.

One date in (plus method-specific extras), full pregnancy timeline out. Not a substitute for an ultrasound dating scan, but the right starting point for everything else.