Due Date Calculator – When Is Your Baby Due?

How Is a Due Date Calculated?

The estimated due date (EDD) is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). Equivalently, you can add 7 days to the LMP date and subtract 3 months. This formula was developed by German obstetrician Franz Karl Naegele in the early 19th century and remains the standard clinical method today.

Practical example: If your last menstrual period started on January 15, your estimated due date is January 15 + 280 days = October 22. Using the shortcut method: January 15 + 7 days = January 22, then subtract 3 months = October 22.

The 280-day gestational period assumes a 28-day menstrual cycle with ovulation occurring on day 14. For cycles longer or shorter than 28 days, the calculation adjusts by adding or subtracting the difference from 14. A woman with a 35-day cycle (ovulation on day 21 rather than day 14) would have an EDD 7 days later than the standard calculation suggests.

According to the American College of Obstetricians and Gynecologists (ACOG), the most accurate method for establishing or confirming gestational age is a first-trimester ultrasound. ACOG recommends that if the ultrasound-based date differs from the LMP-based date by more than 5–7 days in the first trimester, the ultrasound date should be used as the official EDD.

Methods for Determining Your Due Date

Several methods exist for calculating due dates, each with different accuracy levels. Understanding these methods helps you interpret your EDD with appropriate confidence:

First trimester ultrasound is the most accurate dating method because fetal size variation is minimal in early pregnancy — at 8 weeks, nearly all embryos are virtually the same size regardless of genetics. By the third trimester, individual variation in fetal size makes ultrasound dating unreliable for establishing the due date.

Understanding Gestational Age and Pregnancy Timeline

Gestational age is counted from the first day of the last menstrual period — not from conception. This means at the moment of conception (typically day 14 of a 28-day cycle), you're already considered 2 weeks pregnant. A "40-week pregnancy" therefore includes approximately 2 weeks before conception actually occurred.

The WHO and ACOG define pregnancy term categories as follows:

Only about 4–5% of babies are born on their exact due date. According to research published in the BMJ, the median gestational length for first-time mothers is approximately 283 days (40 weeks + 3 days), while subsequent pregnancies average about 280 days. About 80% of babies are born within 2 weeks of the estimated due date.

What to Expect in Each Trimester

Understanding the three trimesters helps expectant mothers prepare physically and emotionally for each stage of pregnancy:

First Trimester (Weeks 1–12): All major organ systems develop during this critical period. The embryo grows from a single cell to approximately 6 cm in length. Common symptoms include morning sickness (affecting 70–80% of pregnancies, according to the CDC), extreme fatigue, breast tenderness, and frequent urination. The risk of miscarriage is highest during this trimester, with approximately 10–15% of known pregnancies ending in miscarriage, most before week 12.

Second Trimester (Weeks 13–27): Often called the "golden period" because nausea usually subsides and energy returns. Fetal movements (quickening) begin around weeks 16–20 — first-time mothers may not notice them until weeks 18–22. The anatomical ultrasound at 18–22 weeks screens for structural abnormalities and often reveals the baby's sex. The baby grows from about 7 cm to 36 cm and begins developing fingerprints, eyebrows, and the ability to hear sounds.

Third Trimester (Weeks 28–40): Rapid fetal growth dominates this phase. The baby gains approximately 250g per week and develops fat stores, mature lungs, and the ability to regulate body temperature. Common maternal symptoms include back pain, shortness of breath, Braxton-Hicks contractions, and difficulty sleeping. Prenatal visits increase to every 2 weeks at 28–36 weeks and weekly after 36 weeks. Group B streptococcus (GBS) screening occurs at 36–37 weeks.

Exercise During Pregnancy: Trimester-by-Trimester Guide

For active women and runners, knowing your due date allows thoughtful planning of modified training phases. The ACOG recommends that pregnant women with uncomplicated pregnancies engage in at least 150 minutes per week of moderate-intensity aerobic activity. Exercise during pregnancy reduces the risk of gestational diabetes by 25–30%, preeclampsia by 40%, and excessive weight gain.

Running-specific guidance: Many recreational runners continue running into the second trimester with appropriate modifications (slower pace, shorter distances, well-supported sports bra). Most runners transition away from running between weeks 28–35 due to pelvic pressure, round ligament discomfort, or simply feeling too uncomfortable. Water running (aqua jogging) is an excellent alternative that maintains cardiovascular fitness without impact stress.

Postpartum return to running: The general guidance is to wait at least 6 weeks postpartum before beginning any running (12 weeks is more conservative). A 2019 consensus statement from UK physiotherapists recommends waiting at least 12 weeks before returning to high-impact exercise and completing a pelvic floor screening first. Factors affecting return include delivery type (vaginal vs. C-section), pelvic floor recovery, breastfeeding status, and sleep quality.

Due Date Myths and Realities

Pregnancy is surrounded by myths and old wives' tales. Understanding the facts helps reduce anxiety and supports evidence-based decision-making:

• Myth: Your due date is precise and your baby should arrive that day. Reality: The EDD is an estimate representing the midpoint of a normal delivery window. Only 4–5% of babies arrive on their exact due date. Think of it as a "due month" rather than a "due date."
• Myth: Going past your due date means something is wrong. Reality: It's common and often normal for first-time mothers to deliver 7–10 days after their EDD. Post-term pregnancy (42 weeks or beyond) does carry increased risks and is typically managed with induction. According to the WHO, induction of labor at 41 weeks is associated with lower rates of perinatal death compared to expectant management.
• Myth: Fetal heart rate predicts gender. Reality: Fetal heart rate varies with gestational age (faster in early pregnancy, slowing slightly as the heart matures) and fetal activity. Studies have found no reliable relationship between fetal heart rate and sex.
• Myth: You can eat whatever you want because you're "eating for two." Reality: Caloric needs increase only modestly — by 0 extra calories in the first trimester, 340 kcal/day in the second, and 450 kcal/day in the third. That's roughly one extra healthy snack, not double portions.
• Myth: Spicy food, walking, or sex will induce labor. Reality: No food, exercise, or activity has been scientifically proven to reliably induce labor. While nipple stimulation has some evidence for promoting contractions in late pregnancy, most folk remedies lack scientific support.

Prenatal Care Schedule and Key Tests

Knowing your due date structures your entire prenatal care schedule. The CDC recommends that women receive regular prenatal care beginning in the first trimester. Here is a typical schedule of visits and screenings:

Women with high-risk pregnancies (advanced maternal age, multiples, gestational diabetes, preeclampsia) will have more frequent monitoring and additional ultrasounds. Always follow your healthcare provider's individualized schedule.

When to Seek Medical Attention During Pregnancy

While this calculator provides an educational estimate of your due date, pregnancy requires professional medical care. Contact your healthcare provider immediately if you experience any of the following at any stage of pregnancy:

• Vaginal bleeding: Especially in the first trimester (may indicate miscarriage or ectopic pregnancy) or third trimester (may indicate placenta previa or abruption)
• Severe or persistent abdominal pain: One-sided pain in early pregnancy could indicate ectopic pregnancy — a medical emergency
• Sudden severe headache or vision changes: May indicate preeclampsia, particularly after 20 weeks
• Reduced fetal movement: After 28 weeks, fewer than 10 movements in 2 hours warrants immediate evaluation
• Fluid leaking from the vagina: May indicate premature rupture of membranes
• Fever above 100.4°F (38°C): Infection during pregnancy requires prompt treatment
• Signs of preterm labor: Regular contractions before 37 weeks, lower back pressure, pelvic pressure

This calculator is an educational tool for general guidance. It is not a medical device and does not replace professional prenatal care. The WHO recommends a minimum of 8 prenatal contacts for a positive pregnancy experience and improved maternal and neonatal outcomes.