Pregnancy Due Date Calculator

About the Pregnancy Due Date Calculator

This calculator estimates your estimated due date (EDD), your current gestational age in weeks and days, your trimester, and a complete table of key milestone dates — all from the first day of your last menstrual period (LMP). It uses Naegele's rule, the same formula your obstetrician or midwife uses as a starting point before ultrasound confirmation.

How due dates are calculated: Naegele's rule

In 1812, German obstetrician Franz Karl Naegele described the rule that still underlies due date estimation today: add 280 days (40 weeks) to the first day of your last menstrual period. The logic is that ovulation typically occurs around day 14 of a 28-day cycle, conception follows ovulation by up to 24 hours, and from conception to birth averages about 266 days (38 weeks). Adding those 14 days of the pre-ovulatory phase to 266 gives the 280-day total counted from LMP.

This calculator applies one important refinement: if your cycle length differs from 28 days, the EDD is shifted accordingly. A 31-day cycle, for example, pushes ovulation to around day 17, so the EDD is adjusted three days later. The formula is:

• EDD = LMP + 280 days + (cycle length − 28 days)

Ultrasound dating, especially in the first trimester, is more accurate than Naegele's rule for women with irregular cycles or uncertain LMP dates. A first-trimester ultrasound at 8–12 weeks can confirm or revise the EDD based on fetal crown-rump length, and providers generally adopt the ultrasound date if it differs from the LMP date by more than 5–7 days.

Why only about 5% of babies are born on their due date

The 40-week EDD is a statistical midpoint, not a deadline. Normal, healthy deliveries occur anywhere from 37 weeks (full term) to 42 weeks (post-term). The actual distribution of delivery dates forms a bell curve centered near the EDD, but with substantial spread — about half of all births occur in the two weeks surrounding the due date. Several factors push delivery earlier or later: fetal size, placental function, maternal health conditions, and simply natural variation in gestation length. This is why providers speak of a due window rather than a precise due date.

Gestational age versus fetal age

Gestational age — what this calculator reports — is counted from the first day of your LMP, which is about two weeks before conception. Fetal age (also called conception age or embryonic age) counts from conception itself and runs approximately two weeks behind gestational age. Medical providers and ultrasound reports universally use gestational age, so this calculator does too. When your provider says "you are 10 weeks pregnant," they mean 10 weeks of gestational age from your LMP.

First trimester development (weeks 1–13)

The first trimester is the most critical period for fetal development. During weeks 3–8, all major organ systems begin forming — a process called organogenesis. The neural tube (which becomes the brain and spinal cord) closes by week 6. The heart begins beating around week 5–6. By week 8, all essential organs are present in rudimentary form, the embryo becomes a fetus, and arm and leg buds are forming. By week 12, the fetus has fingers, toes, and detectable facial features. The placenta takes over hormone production from the corpus luteum around week 10.

Common symptoms in the first trimester include nausea and vomiting (affecting up to 80% of pregnant women, often called "morning sickness" though it can occur at any time), fatigue, breast tenderness and swelling, frequent urination, and food aversions or cravings. These symptoms typically peak around weeks 8–10 and ease by weeks 12–14 for most women.

Prenatal care schedule

Prenatal appointments follow a predictable schedule for low-risk pregnancies. The first appointment typically occurs at 8–10 weeks and includes confirmation of pregnancy, LMP review, blood pressure and weight baseline, blood type and Rh factor, complete blood count, sexually transmitted infection screening, rubella and varicella immunity, urine analysis, and in many practices a first-trimester ultrasound. After that initial visit, appointments are typically monthly (every 4 weeks) through 28 weeks, biweekly (every 2 weeks) from 28–36 weeks, and weekly from 36 weeks until delivery. High-risk pregnancies involve more frequent monitoring.

Nutrition in pregnancy

Nutritional needs increase substantially during pregnancy. Key nutrients include:

• Folic acid: 400–800 mcg daily, ideally starting before conception. Reduces risk of neural tube defects by up to 70%. Found in leafy greens, legumes, and fortified cereals.
• DHA (omega-3 fatty acid): 200–300 mg daily supports fetal brain and eye development. Found in low-mercury fatty fish (salmon, sardines) or algae-based supplements.
• Iron: 27 mg daily (increased from 18 mg pre-pregnancy) to support expanded maternal blood volume and fetal iron stores.
• Calcium: 1,000 mg daily for fetal bone development; 1,300 mg for teens.
• Iodine: 220 mcg daily for fetal thyroid development.

Water intake also increases substantially during pregnancy — most providers recommend 8–12 cups of fluid daily, and a pregnancy water intake calculator can give you a personalized target based on your body weight and activity level.

Foods to avoid during pregnancy include: raw or undercooked fish and shellfish (risk of parasites and mercury); deli meats and hot dogs unless heated until steaming (risk of listeria); unpasteurized cheeses and juices (listeria and other pathogens); raw or undercooked eggs; high-mercury fish (shark, swordfish, king mackerel, tilefish); liver in large quantities (excess vitamin A); alcohol (no safe level is established); and excessive caffeine (limit to under 200 mg/day).

Second trimester overview (weeks 14–27)

Often called the "golden trimester" because first-trimester nausea typically subsides and the discomforts of late pregnancy have not yet begun. Key events include the anatomy scan ultrasound at around 20 weeks, which checks fetal anatomy for structural abnormalities and, if desired by the parents, can reveal the sex. Fetal movement (quickening) is first felt between 18 and 22 weeks — earlier in women who have had a previous pregnancy. The fetus grows from about 4 inches to 14 inches and from 2 ounces to nearly 2 pounds during this trimester. Braxton Hicks contractions (practice contractions) may begin in the second trimester.

Third trimester and preparing for delivery (weeks 28–40+)

In the third trimester, the fetus gains most of its birth weight, lung maturity develops, and the baby moves into a head-down position in preparation for birth. Key third-trimester tests and appointments include:

• Gestational diabetes screening: The glucose challenge test is performed at 24–28 weeks. Gestational diabetes affects approximately 6–9% of pregnancies and is managed with diet, exercise, and sometimes insulin.
• Group B Streptococcus (GBS) test: A vaginal and rectal swab performed at 35–37 weeks. GBS is a common bacterium; about 25% of healthy adults carry it. If positive, antibiotics are given during labor to protect the newborn.
• RhoGAM for Rh-negative mothers: If your blood type is Rh-negative and your baby may be Rh-positive, an injection of Rh immunoglobulin (RhoGAM) is given at 28 weeks and again within 72 hours of delivery to prevent Rh sensitization in future pregnancies.
• Fetal position: By 36 weeks most babies are head-down (cephalic presentation). Breech position (feet or bottom first) affects about 3–4% of full-term pregnancies and may lead to a recommendation for external cephalic version (ECV) or cesarean delivery.

Signs of preterm labor

Preterm labor before 37 weeks requires immediate medical attention. Warning signs include: regular uterine contractions (more than 4 per hour) before 37 weeks; low, dull backache; pelvic pressure or a feeling that the baby is pushing down; abdominal cramps with or without diarrhea; a change in vaginal discharge (watery, mucus-like, or bloody); and fluid leaking from the vagina. If you experience any of these before 37 weeks, contact your provider or go to labor and delivery immediately.

Calculating weeks vs. months of pregnancy

Pregnancy is measured in weeks, not months, because fetal development milestones and screening tests are calibrated to gestational weeks. The common claim that "pregnancy lasts 9 months" is a simplification — it actually spans closer to 10 lunar months (10 × 4 weeks = 40 weeks). The disconnect causes confusion: if you divide 40 weeks by 4.33 (the average weeks per calendar month), you get about 9.2 calendar months. Neither "9 months" nor "10 months" is precisely correct; "40 weeks from LMP" is the most accurate way to state it.

Conception and ovulation timing

In a standard 28-day cycle, ovulation occurs around day 14 (counting from the first day of the period). The egg is viable for fertilization for only 12–24 hours after ovulation, but sperm can survive in the reproductive tract for up to 5 days. This creates a fertile window of approximately 5–6 days around ovulation (days 10–15 of a 28-day cycle). Conception most often occurs 1–2 days before or on the day of ovulation. In longer cycles, ovulation is delayed proportionally — in a 35-day cycle, ovulation typically falls around day 21.

IVF dating

For pregnancies conceived through in vitro fertilization (IVF), the transfer date provides a more precise anchor for gestational age than LMP:

• Day-3 embryo transfer: Add 266 days to get the EDD (equivalent to LMP + 280 days − 3 days).
• Day-5 blastocyst transfer: Add 261 days (LMP + 280 − 5 days).

IVF due dates are generally more reliable than LMP-based estimates because the date of fertilization is known precisely.

Key Milestone Reference Tables

Table 1: Gestational week, developmental milestone, and prenatal test/appointment

Table 2: Trimester overview — weeks, milestones, and common symptoms

Table 3: Cycle length, Naegele's rule adjustment, and example EDD if LMP is October 1

Frequently Asked Questions