Pregnancy Due Date & Gestational Age Calculator

Understanding Pregnancy Due Date Calculation and Gestational Age

Determining an accurate estimated due date (EDD) and tracking gestational age are fundamental to prenatal care, delivery planning, and fetal development monitoring. Pregnancy is typically dated from the first day of the last menstrual period (LMP), even though actual conception occurs approximately two weeks later (at ovulation). This dating convention, standardized by obstetricians and gynecologists worldwide, results in a normal pregnancy lasting 280 days or 40 weeks from LMP. However, actual delivery dates vary considerably: only about 5% of pregnancies deliver exactly on the due date, and 90% deliver within two weeks of the calculated date. Understanding how due dates are calculated, what factors influence delivery timing, and how to interpret dates across different pregnancy dating methods is important for expectant parents and healthcare providers.

The most common dating method is based on the last menstrual period (LMP). Naegele rule, developed in 1812 and still the standard clinical tool, calculates due date by subtracting three months from the LMP and adding one year and fourteen days. For example, if LMP was May 5, subtract three months (February 5), add one year (February 5 next year), and add 14 days (February 19). This formula produces a due date 280 days (40 weeks) after LMP. However, Naegele rule assumes a regular 28-day cycle; for women with longer or shorter cycles, the due date should be adjusted. A woman with a 35-day cycle (7 days longer than 28) ovulates about 7 days later, so conception occurs later, and the due date should be approximately one week later than Naegele calculation. Conversely, women with 21-day cycles ovulate earlier, and the due date should be moved earlier.

Ultrasound dating is increasingly accurate, particularly in early pregnancy. A dating scan (typically performed at 8-14 weeks gestation) measures fetal size and compares it to population standards. In the first trimester, ultrasound dating accuracy is ±3-5 days. By the second trimester (15-20 weeks), accuracy decreases to ±7-10 days, and in the third trimester (>20 weeks), accuracy drops to ±2-3 weeks. If an early ultrasound (before 13 weeks) shows a different due date than LMP-based dating by more than 5 days, the ultrasound date is typically used because it is more reliable. Discrepancies of more than 5 days suggest either incorrect LMP recall or variation in individual fetal growth rates.

Pregnancy is divided into three trimesters, each with distinct fetal development milestones and maternal changes. The first trimester (weeks 1-13) encompasses major organ formation, from neural tube development to functional heart, lungs, and kidneys. By the end of week 12, the fetus is approximately 2-3 inches long and weighs less than 1 ounce. Many miscarriages occur during this period; once a viable pregnancy is confirmed on ultrasound (around 8-9 weeks), miscarriage risk decreases substantially. The second trimester (weeks 14-27) is characterized by rapid growth and increasing fetal awareness. By week 20 (mid-pregnancy), the fetus is approximately 10 inches long and can be clearly visualized on ultrasound. Fetal movements (quickening) typically become noticeable to the mother around weeks 16-20. By the end of the second trimester (week 27), the fetus weighs approximately 2 pounds and has improving chances of survival if born prematurely.

The third trimester (weeks 28-40) involves continued growth and positioning for delivery. By week 36, the fetus is considered full-term in most contexts, and delivery is safe without increased neonatal complications from prematurity. Between weeks 37-40, the fetus typically moves into vertex (head-down) position in preparation for vaginal delivery. The average term newborn weighs 7-8 pounds and measures 19-21 inches. Pregnancies delivered before 37 weeks are classified as preterm; those delivered after 42 weeks are classified as post-term and carry increased risk for placental insufficiency and fetal complications.

MathML Formula for Naegele Rule:

This simplifies to: LMP + 280 days (or +40 weeks).

Worked Example: A woman's last menstrual period began on June 10, 2024. Using Naegele rule: subtract 3 months (March 10), add 1 year (March 10, 2025), add 14 days (March 24, 2025). Estimated due date is March 24, 2025. She is currently at 20 weeks gestation (mid-pregnancy) if today is October 10, 2024, and expects delivery around March 24-31, 2025 (due dates typically have a 1-week window).

Trimester milestones and typical characteristics:

Trimester	Weeks	Key Milestones	Typical Fetal Weight
First	1–13	Organ formation, heartbeat visible by 6 weeks	< 1 oz at week 13
Second	14–27	Rapid growth, fetal movement felt, viability threshold	~2 lbs at week 27
Third	28–40	Final growth, positioning for birth, term reached at 37 weeks	~7.5 lbs at delivery

Factors that influence actual delivery date include parity (first pregnancies tend to deliver slightly after the due date), maternal age (older mothers slightly more likely to deliver on or before due date), fetal sex (male fetuses tend to be born slightly later), and various maternal and placental factors. Induction of labor is typically offered at 39 weeks for most pregnancies; labor is indicated earlier for pregnancies with complications (preeclampsia, gestational diabetes with poor control, fetal growth restriction, etc.) and later only in specific low-risk scenarios.

Limitations and Assumptions: This calculator provides estimates and should not replace professional obstetric care. Due date is an estimate; actual delivery may vary by ±2 weeks. LMP-based dating assumes regular cycles and accurate recall; if uncertain about LMP, ultrasound dating is more reliable. This calculator does not account for assisted reproductive technology (ART) cycles, which require specific calculation based on fertilization or embryo transfer date. Multiple pregnancies (twins, triplets) typically deliver earlier than 40 weeks; consult obstetric provider for specific timelines. Medical conditions, previous pregnancy complications, and individual patient factors may warrant modification of standard dating and delivery timing. Always consult with your obstetric provider for personalized dating and delivery planning.