Pregnancy Due Date Calculator

How Is a Due Date Calculated? Naegele's Rule Explained

The most widely used method for calculating an estimated due date (EDD) is Naegele's Rule, named after German obstetrician Franz Karl Naegele. The formula: take the first day of the last menstrual period (LMP) and add 280 days (40 weeks). A shortcut: add 1 year, subtract 3 months, and add 7 days. The American College of Obstetricians and Gynecologists (ACOG) endorses this method as the standard approach when ultrasound is not available. The 40-week count begins from the LMP — not from conception — which means the first two weeks of “pregnancy” actually predate fertilization.

Worked Example: Emma's Due Date

Emma's last menstrual period began on January 15, 2026. Using Naegele's Rule:

• Start date: January 15, 2026
• Add 1 year: January 15, 2027
• Subtract 3 months: October 15, 2026
• Add 7 days: October 22, 2026 — Emma's estimated due date

On March 13, 2026, Emma is 8 weeks and 3 days pregnant (57 days since LMP). She is in the first trimester, approaching her NIPT screening window. Her OB may adjust this date by a few days after her first-trimester ultrasound if her cycle is slightly longer or shorter than 28 days.

If Emma's cycle is 31 days (longer than the 28-day assumption), she likely ovulated around day 17 rather than day 14. Her conception date was 3 days later than assumed, which means her EDD might be adjusted to October 25, 2026 after ultrasound confirmation. First-trimester ultrasound before 14 weeks is the most accurate dating method available, typically within ±5 days.

Trimester Milestones: Week by Week

Pregnancy is divided into three trimesters, each with distinct developmental milestones, physical changes for the mother, and key medical tests. Source: March of Dimes: Stages of Pregnancy:

Prenatal Visit Schedule: What Happens at Each Appointment

A typical low-risk US pregnancy involves 10–15 prenatal visits. Here is the standard ACOG-recommended schedule:

Why Only 5% of Babies Are Born on Their Due Date

The EDD is a statistical midpoint, not a prediction. Research published in the BJOG: International Journal of Obstetrics and Gynaecology found that only approximately 4–5% of pregnancies deliver on the exact EDD. The majority of births cluster within two weeks on either side:

• 70% of babies are born within 10 days of the EDD
• 90% are born within 2 weeks before or after
• First-time mothers (nulliparas) deliver on average 4–5 days after the EDD
• Women who have previously given birth tend to deliver closer to or before the EDD

This variability is driven by the biological initiation of labor, which depends on fetal lung maturity signaling, hormonal cascades (oxytocin, prostaglandins), and cervical readiness — all of which occur on the fetus's timeline, not the calendar. The EDD is best understood as the center of a 5-week window (weeks 37–42) during which delivery is considered normal.

ACOG defines the following delivery windows (source: March of Dimes): preterm (before 37 weeks), early term (37–38 weeks), full term (39–40 weeks, optimal), late term (41 weeks), post-term (42+ weeks, induction typically recommended).

IVF Due Date Calculation: A More Precise Starting Point

For pregnancies achieved through in vitro fertilization (IVF), the fertilization date is known precisely, making EDD calculation more accurate than LMP-based methods. ACOG guidelines for IVF dating:

• Day 3 embryo transfer: EDD = transfer date + 263 days
• Day 5 blastocyst transfer: EDD = transfer date + 261 days

These formulas work backward from the known fertilization date to establish an equivalent “LMP date,” then apply the standard 280-day calculation. Despite more precise initial dating, IVF pregnancies have the same delivery variability as naturally conceived pregnancies — only 4–5% still deliver exactly on the EDD.

How Due Dates Are Calculated: Naegele's Rule

The standard method used by obstetricians worldwide is Naegele's rule, named after German obstetrician Franz Karl Naegele who formalized it in 1830. The calculation: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Alternatively, add 1 year, subtract 3 months, and add 7 days.

Why 280 days? Ovulation typically occurs around day 14 of a 28-day cycle, and fertilization follows within 12–24 hours. By counting from LMP rather than conception, the 40 weeks includes approximately 2 weeks before the egg was fertilized — which is why a “40-week pregnancy” represents only about 38 weeks of actual fetal development. The American College of Obstetricians and Gynecologists (ACOG) endorses Naegele's rule as the standard method for establishing gestational age when ultrasound is unavailable.

Real Scenario: Sarah's Due Date, Step by Step

Sarah's last menstrual period began on January 15, 2026. Using Naegele's rule:

• Start date: January 15, 2026
• Add 1 year: January 15, 2027
• Subtract 3 months: October 15, 2026
• Add 7 days: October 22, 2026

Sarah's estimated due date (EDD) is October 22, 2026. On today's date (March 13, 2026), she is approximately 8 weeks and 1 day pregnant (57 days since LMP). She is in her first trimester, approaching her first major prenatal milestone — the 10–13 week NIPT screening.

Sarah has a 29-day cycle rather than the standard 28. Her actual ovulation likely occurred around day 15 — one day later than the formula assumes — so her OB may adjust the EDD by one day after her first ultrasound. Women with cycles significantly longer or shorter than 28 days routinely get LMP-based EDDs adjusted by first-trimester ultrasound.

Why Your “Due Date” Is Really a Due Month

The EDD is a statistical midpoint — not a prediction. Research published in BJOG: An International Journal of Obstetrics and Gynaecology found that only approximately 4.4% of pregnancies deliver exactly on the EDD. About 70% deliver within 10 days of the EDD, and roughly 90% deliver within 2 weeks before or after. First-time mothers (nulliparas) tend to deliver on average 4–5 days after their EDD; women who have previously given birth (multiparas) tend to deliver slightly closer to or before the EDD.

ACOG defines a range of normal delivery windows. Source: March of Dimes: Full-Term Pregnancy:

Trimester Breakdown: Baby Size, Milestones, and Key Tests

Pregnancy is divided into three trimesters, each with distinct developmental priorities, physical changes, and medical appointments:

Source: March of Dimes: Stages of Pregnancy and CDC Pregnancy Resources.

Prenatal Appointment Timeline: What Happens When

A typical low-risk pregnancy in the US involves 10–15 prenatal visits over 40 weeks. Here is the standard ACOG-recommended schedule:

IVF Due Date Calculation: When the Date Is Known Precisely

For IVF pregnancies, the fertilization date is documented, making dating more precise than LMP calculations. ACOG guidelines:

• Day 3 embryo transfer: EDD = transfer date + 263 days (38 weeks − 3 days from the “equivalent conception date” that is 17 days before transfer)
• Day 5 blastocyst transfer: EDD = transfer date + 261 days (equivalent LMP is 19 days before transfer)

IVF pregnancies have more precise initial EDDs, but the same delivery variability applies — only 4–5% of IVF babies arrive exactly on the EDD either. The biological variability of labor initiation is independent of how accurately conception is dated.

What to Prepare Each Trimester: A Practical Timeline

First Trimester (Weeks 1–12): Foundation

• Start or confirm prenatal vitamins with folic acid (400–800 mcg/day recommended by CDC)
• Schedule your first OB appointment; confirm your health insurance covers prenatal care
• Avoid alcohol, tobacco, raw fish, high-mercury fish, unpasteurized products, and excess caffeine (<200mg/day)
• Research NIPT testing options; understand what chromosomal screening can and cannot tell you
• Begin research on birth settings (hospital, birth center) and care providers (OB, midwife)

Second Trimester (Weeks 13–26): Planning

• Schedule the anatomy ultrasound (weeks 18–20) and decide about sex reveal
• Begin maternity leave planning — notify employer, review FMLA eligibility (12 weeks unpaid for qualifying employees at companies with 50+ employees)
• Start researching child care options (waitlists for quality infant care can be 6–18 months)
• Register for childbirth education class; most recommend completing by week 34
• Begin baby gear research: car seat (must be installed before hospital discharge), crib or bassinet, feeding supplies

Third Trimester (Weeks 27–40+): Preparation

• Pack hospital bag by week 36 (preterm labor is possible before this)
• Install car seat and get it inspected at a certified check station
• Finalize pediatrician selection; interview 2–3 practices before delivery
• Complete hospital pre-registration to streamline admission
• Create a birth preference document (not a rigid “birth plan” but a communication tool for preferences)
• Review newborn care basics: safe sleep (back-only on firm surface, no loose bedding — CDC safe sleep guidelines)

Warning Signs: When to Call Your Doctor

Most pregnancies progress without complication, but some symptoms require immediate medical attention. Per ACOG Warning Signs During Pregnancy, contact your provider immediately if you experience:

• Heavy vaginal bleeding (more than spotting) at any point in pregnancy
• Severe or persistent abdominal or pelvic pain
• Signs of preeclampsia: sudden severe headache, visual disturbances, rapid swelling of face/hands/feet, upper abdominal pain
• Fever above 100.4°F (38°C)
• Decreased fetal movement after week 28 (fewer than 10 kicks in 2 hours)
• Signs of preterm labor before 37 weeks: regular contractions, pelvic pressure, low back pain, fluid leakage

Pregnancy Nutrition and Weight Gain Guidelines

Pregnancy nutrition directly affects fetal development, birth weight, and maternal health outcomes. The CDC and Institute of Medicine provide clear guidelines on healthy weight gain and nutrient priorities:

• Folic acid: 400–800 mcg/day starting at least 1 month before conception reduces neural tube defect risk by 50–70%
• Iron: 27 mg/day (up from 18mg non-pregnant); supports expanded blood volume and fetal iron stores
• Calcium: 1,000 mg/day; critical for fetal bone development; if maternal intake is insufficient, fetus draws from maternal bone
• DHA (omega-3): 200–300 mg/day; essential for fetal brain and eye development; found in salmon, sardines, and prenatal supplements
• Vitamin D: 600 IU/day; many OBs now recommend 1,000–2,000 IU given widespread deficiency

Recommended weight gain during pregnancy depends on pre-pregnancy BMI. Women who start at a healthy BMI (18.5–24.9) should gain 25–35 lbs total. Underweight women (BMI under 18.5) should gain 28–40 lbs. Overweight women (BMI 25–29.9) should aim for 15–25 lbs, and obese women (BMI 30+) should limit gain to 11–20 lbs to minimize complications. Most weight gain occurs in the second and third trimesters: roughly 1 lb/week during weeks 14–40.

Common Pregnancy Questions: Dates, Symptoms, and What's Normal

Can I calculate my due date if I don't know my LMP?

Yes. An early first-trimester ultrasound (typically performed between 8–14 weeks) measures the crown-rump length (CRL) of the embryo to estimate gestational age. This method is highly accurate within ±5–7 days before 14 weeks. If your cycles are irregular, very long, or very short, ultrasound dating is more reliable than Naegele's rule from LMP. Most providers perform a dating ultrasound at the first prenatal visit if LMP is uncertain.

What is the difference between gestational age and fetal age?

Gestational age is measured from the first day of your last menstrual period (LMP) — the standard in all medical documentation. A 10-week pregnancy is 10 gestational weeks from LMP. Fetal age (also called embryonic age) is measured from conception, which typically occurs about 2 weeks after LMP. So a fetus at 10 gestational weeks is approximately 8 weeks of actual fetal development. Healthcare providers always use gestational age; it is the number this calculator displays.

Is morning sickness a sign of a healthy pregnancy?

Nausea and vomiting (“morning sickness,” which occurs any time of day) affects 70–80% of pregnant women and typically peaks between weeks 8–10. Studies published in JAMA Internal Medicine suggest that moderate nausea is associated with lower rates of miscarriage — it is thought to reflect healthy placental hormone production. However, hyperemesis gravidarum (severe, persistent vomiting causing dehydration and weight loss) affects 0.5–2% of pregnancies and requires medical treatment. Track nausea patterns and report any inability to keep fluids down for 24+ hours to your provider.

Frequently Asked Questions

How is the due date calculated?

Using Naegele's rule (endorsed by ACOG): LMP first day + 280 days. Quick method: add 1 year, subtract 3 months, add 7 days. If conception date is known, add 266 days. A first-trimester ultrasound before 14 weeks is the most accurate dating method and may adjust the LMP-based EDD — particularly for women with irregular cycles.

What are the three trimesters of pregnancy?

First trimester: weeks 1–12. All major organs form; heartbeat detectable by week 6–7; highest miscarriage risk. Second trimester: weeks 13–26. Often most comfortable period; anatomy scan; quickening (first felt movement). Third trimester: week 27 through birth. Rapid fetal growth; lung maturation; delivery preparation. Full term is 39–40 weeks.

When is a baby considered full term?

ACOG defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term (37–38 weeks) carries higher risk of breathing problems, feeding difficulties, and NICU admission than full term. Most providers recommend waiting until at least 39 weeks for elective induction unless there is a medical indication.

How accurate is an estimated due date (EDD)?

Only ~4–5% of babies are born exactly on the EDD. About 90% are born within 2 weeks before or after. First-time mothers tend to deliver 4–5 days after the EDD on average. An early first-trimester ultrasound is more accurate than LMP calculation alone — particularly when LMP is uncertain or cycles are irregular.

Understanding Your Due Date: Key Takeaways

Your estimated due date is the single most useful organizing number in your pregnancy — it anchors your prenatal visit schedule, determines which screening windows apply, and helps your care team monitor your baby's growth against developmental milestones. But it is an estimate, not a guarantee. Here are the most important points to carry forward:

• Your EDD is calculated from LMP and may be adjusted by first-trimester ultrasound — accept the ultrasound-adjusted date if your OB offers one before 14 weeks; it is more accurate.
• A “normal” delivery window spans weeks 37–42. If you reach 41 weeks, most providers increase monitoring and discuss induction options; standard practice is induction at 41–42 weeks to reduce post-term complications.
• Take folic acid (400–800 mcg/day) starting at least one month before conception and through the first trimester — this single supplement reduces neural tube defect risk by 50–70%.
• Know your pregnancy warning signs: heavy bleeding, severe abdominal pain, sudden severe headache with vision changes, and signs of preterm labor (regular contractions before 37 weeks) all warrant immediate contact with your provider.

Track your baby's gestational age and development timeline with this calculator, and use our Age Calculator for other date-based calculations. Our BMI Calculator can help contextualize pregnancy weight gain guidelines based on pre-pregnancy BMI.

Related Calculators