Pregnancy Due Date Calculator

This pregnancy due date calculator uses Naegele's Rule — the standard method recommended by the American College of Obstetricians and Gynecologists (ACOG) — to estimate your baby's due date. Developed around 1850 by German obstetrician Dr. Franz Karl Naegele, the rule adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). It assumes a standard 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is longer or shorter than 28 days, our calculator adjusts the due date accordingly.

4 Methods to Calculate Your Estimated Due Date

We offer four calculation methods to give you the most accurate estimate possible:

1. Last Menstrual Period (LMP) — The most commonly used method. Enter the first day of your last period and your average cycle length. The calculator applies Naegele's Rule with cycle-length adjustment.
2. Conception Date — Adds 266 days (38 weeks) from your known conception date. Ideal for women who track ovulation using basal body temperature, OPKs (ovulation predictor kits), or who conceived through IUI (intrauterine insemination).
3. IVF Transfer Date — The most precise method available. For a Day 3 embryo transfer, 263 days are added. For a Day 5 blastocyst transfer, 261 days are added (the embryo is 2 days more developed at transfer).
4. Ultrasound Scan — Uses the gestational age measured during an ultrasound (typically via crown-rump length in the first trimester) to back-calculate your LMP and due date. ACOG considers first-trimester ultrasound dating (6–13 weeks) the most accurate method, typically within ±5–7 days.

How Accurate Is Your Due Date?

Only about 5% of babies are born on their exact estimated due date. Approximately 80% are born between 37 and 42 weeks. ACOG defines pregnancy timing with these official categories:

Can Your Due Date Change?

Your due date can change after an ultrasound if there's a significant discrepancy between the LMP-based date and the ultrasound measurement. ACOG guidelines for redating:

• 1st Trimester (up to 13w 6d): Redate if ultrasound differs by more than 7 days
• Early 2nd Trimester (14–15 weeks): Redate if difference exceeds 10 days
• Mid 2nd Trimester (16–21 weeks): Redate if difference exceeds 14 days
• Late pregnancy (22+ weeks): Redate if difference exceeds 21 days

If your OB-GYN changes your due date, it's because ultrasound-measured fetal size provides stronger evidence of gestational age than period-based calculation, especially for women with irregular cycles.

Understanding Trimesters: What Happens When

Gestational Age vs. Fetal Age

Gestational age is measured from the first day of your last menstrual period — it includes approximately 2 weeks before conception actually occurred. Fetal age (embryonic age) starts from the actual date of conception and is about 2 weeks less than gestational age.

When to See Your OB-GYN: US Prenatal Visit Schedule

The standard prenatal care schedule recommended by ACOG:

At each visit, your OB-GYN checks blood pressure, weight gain, fundal height, fetal heart rate, and urine.

Factors That Can Affect Your Due Date

• Irregular menstrual cycles — The most common cause of inaccurate LMP-based dates
• First-time mothers (nulliparas) — Tend to deliver 1–5 days past their due date on average
• Previous births (multiparas) — Tend to deliver slightly earlier
• Multiple pregnancies — Twins average 36 weeks; triplets average 32 weeks
• Maternal age over 35 — Slightly higher rates of post-term pregnancy
• Gestational diabetes — May lead to early induction
• Family history — Pattern of early or late delivery may run in families