The due date is primarily calculated using Naegele's Rule: start with the first day of your last menstrual period (LMP), add 280 days (40 weeks). This calculator also adjusts for your individual cycle length. If your cycle is longer or shorter than the standard 28 days, the due date shifts accordingly—add one day for each day your cycle exceeds 28 days, or subtract one day for each day it's shorter. For example, a 32-day cycle adds 4 extra days (280 + 4 = 284 days from LMP).

In medical practice, pregnancy is measured from the first day of your last menstrual period (LMP), not from the date of conception. This is because the exact conception date is often unknown, while the LMP date is usually easier to recall. For a typical 28-day cycle, ovulation and conception occur around day 14—meaning by the time you conceive, you're already considered "2 weeks pregnant." This standardized method ensures consistency across all pregnancy dating.

Only about 4–5% of babies are born exactly on their due date. The due date is best understood as an estimate within a range—most babies arrive between 37 and 42 weeks (considered full-term). Factors like irregular cycles, uncertain LMP dates, and individual variations all affect accuracy. Early ultrasound dating (before 20 weeks) is considered the gold standard for confirming or adjusting the due date. Always consult your healthcare provider for personalized guidance.

Pregnancy is divided into three trimesters, each lasting approximately 13–14 weeks:

• First Trimester (Weeks 1–13): Major organ development occurs. Common symptoms include fatigue, nausea, and breast tenderness. The risk of miscarriage is highest during this period.
• Second Trimester (Weeks 14–26): Often called the "honeymoon period"—energy returns, nausea subsides, and the baby's movements become noticeable. Major anatomy scans are performed.
• Third Trimester (Weeks 27–40): Rapid weight gain for the baby, final lung development, and preparation for birth. Discomfort may increase as the due date approaches.

The standard Naegele's Rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer (e.g., 35 days), ovulation likely occurs later (around day 21), so the actual conception happens later, pushing the due date forward. Conversely, a shorter cycle (e.g., 21 days) means earlier ovulation (around day 7) and an earlier due date. Our calculator adjusts the due date by ±1 day for each day your cycle differs from 28 days. If you're unsure about your cycle length, using 28 days is a reasonable default.

If you can't recall your LMP date, an early ultrasound (usually performed between 8–14 weeks) is the most reliable way to date your pregnancy. The ultrasound measures the crown-rump length (CRL) of the embryo/fetus, which provides a very accurate gestational age estimate. Some women also use ovulation tracking data or known conception dates if available. If you know your conception date, you can estimate LMP by subtracting approximately 14 days (for a 28-day cycle) from the conception date.

The American College of Obstetricians and Gynecologists (ACOG) defines full-term pregnancy with more precision:

• Early Term: 37 weeks 0 days – 38 weeks 6 days
• Full Term: 39 weeks 0 days – 40 weeks 6 days
• Late Term: 41 weeks 0 days – 41 weeks 6 days
• Postterm: 42 weeks 0 days and beyond

Babies born at full term (39–40 weeks) generally have the best health outcomes. Delivery before 37 weeks is considered preterm.

For IVF pregnancies, dating works differently. Instead of using LMP, the due date is calculated from the embryo transfer date and the age of the embryo at transfer. For a day-5 embryo transfer, the due date is approximately 261 days after the transfer date. For a day-3 transfer, it's about 263 days. If you conceived via IVF, consult your fertility specialist for the most accurate due date based on your specific treatment protocol.