Pregnancy Due Date Calculator
Estimate your pregnancy due date, gestational age, and trimester from the first day of your last menstrual period.
How to use this pregnancy due date calculator
- 1Enter days since the first day of your last menstrual period.
- 2Enter your average cycle length — 28 days is average.
- 3The calculator shows gestational age, trimester, days remaining, and developmental stage.
- 4Only about 5% of babies are born on their exact due date — 37–42 weeks is the normal delivery window.
- 5An early ultrasound (8–12 weeks) provides the most accurate due date — use this as an estimate only.
How it's calculated
Due date = LMP + 280 days (Naegele's rule), adjusted for cycle length. Gestational age counts from LMP first day.
About the Pregnancy Due Date Calculator
The estimated due date is one of the most anticipated numbers in a pregnancy, yet it represents a statistical midpoint rather than a precise prediction. Naegele's rule — adding 280 days (40 weeks) to the first day of the last menstrual period — has been the standard dating method since the 19th century and remains widely used despite its simplifying assumptions.
The 40-week count begins from the last menstrual period, not from conception. This means the first two weeks of gestational age occur before conception has taken place. Conception typically occurs around day 14 of a 28-day cycle, making actual embryonic age approximately 2 weeks less than gestational age. When healthcare providers say a patient is 12 weeks pregnant, the embryo has been developing for approximately 10 weeks.
The importance of early ultrasound for accurate dating cannot be overstated. Crown-rump length measured at 8–12 weeks has an accuracy of ±5 days, while the LMP method has accuracy of ±2 weeks for regular cycles and wider variation for irregular cycles. In women who conceived through fertility treatments, the exact fertilization date is known and provides the most accurate dating possible.
For expectant parents, gestational age milestones provide context for fetal development. The embryonic period (weeks 3–8 post-conception) is when all major organ systems are established — this is the period of highest susceptibility to teratogens (substances causing birth defects), which is why early prenatal care and avoidance of alcohol, certain medications, and other harmful exposures is most critical in the first trimester.
Frequently asked questions
How accurate is an LMP-based due date?
LMP-based due date calculation assumes a 28-day cycle with ovulation on day 14. For women with regular 28-day cycles, this is reasonably accurate — typically within 1–2 weeks of actual delivery. For women with irregular cycles or significant variation in cycle length, accuracy decreases substantially. An early ultrasound (8–12 weeks) measures fetal crown-rump length and provides a due date accurate to within 5–7 days, making it the gold standard for dating. When LMP and ultrasound dates disagree by more than 5–7 days, obstetric guidelines generally recommend using the ultrasound date.
What are the three trimesters of pregnancy?
First trimester (weeks 1–12): critical organ development; the embryo transitions to fetus. Risk of miscarriage is highest here (approximately 10–20% of known pregnancies). Second trimester (weeks 13–26): often the golden period — morning sickness typically improves, energy returns, and fetal movements become perceptible. The fetus grows from about 3 inches to over 14 inches. Third trimester (weeks 27–40+): rapid fetal growth and lung maturation, gaining approximately 0.5 lb per week in final weeks. Delivery before 37 weeks is preterm; 39–40 weeks is full term; beyond 42 weeks is post-term.
Why do only 5% of babies arrive on the due date?
The 40-week due date is a statistical average, not a guarantee. Spontaneous labor is influenced by complex hormonal signals between fetus, placenta, and uterus that are not fully understood. Normal delivery occurs anywhere from 37 to 42 weeks of gestation. Approximately 50% of first-time mothers deliver after their due date. Induced or cesarean deliveries before 42 weeks account for a significant portion of deliveries in modern obstetric care. The due date is best understood as the midpoint of a delivery window rather than a deadline.
What prenatal care is recommended each trimester?
First trimester: establish care with an OB/midwife, initial blood work, Rh factor testing, STI screening, genetic carrier screening if desired, and nuchal translucency ultrasound (weeks 11–13). Prenatal vitamins with folic acid (400–800 mcg) should start as early as possible, ideally before conception. Second trimester: anatomy ultrasound (18–20 weeks) and gestational diabetes screening (24–28 weeks). Third trimester: Group B strep testing (35–37 weeks), fetal position assessment, and birth planning. Visit frequency increases from monthly to weekly as the due date approaches.