Due Date Calculator
Estimate your baby's due date and find out how many weeks pregnant you are based on your last period, ovulation date, or conception date.
Pregnancy Stages
Weeks 1–4
Implantation & early development
Weeks 5–12
Organs form, heartbeat begins
Weeks 13–27
Growth & movement
Weeks 28–40
Maturation & full term
Most pregnancies last 38–42 weeks
Select Calculation Method
Range: 21–35 days. Default: 28.
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How Is a Pregnancy Due Date Calculated?
The standard method for calculating a due date is Naegele's rule, developed in the 19th century and still widely used today. It adds 280 days — 40 weeks — to the first day of the last menstrual period (LMP). This method assumes a 28-day cycle with ovulation occurring on day 14. For cycles shorter or longer than 28 days, the due date is adjusted by the difference.
The reason we count from the LMP rather than from conception or ovulation is practical: the exact date of ovulation and conception are often unknown, while most people know when their last period started. Pregnancy is therefore measured in 'gestational weeks' which include the roughly two weeks before conception.
When ovulation or conception is known — through ovulation tracking, IVF records, or early ultrasound — the due date can be calculated more directly: 266 days (38 weeks) from ovulation or conception. This is equivalent to 40 weeks from the LMP on a standard 28-day cycle.
How Accurate Are Due Date Calculators?
Calculator-based due dates are estimates. Research shows that most babies are born between 37 and 42 weeks of gestation, and only about 5% arrive on their calculated due date. The due date is better understood as the midpoint of a several-week window rather than a fixed target.
First-trimester ultrasound is more precise than LMP-based dating, particularly when cycle length is irregular or the LMP is uncertain. Ultrasound measures the baby's crown-rump length (CRL) and compares it to known growth standards to estimate gestational age, typically to within ±5–7 days before 12 weeks, as described by ACOG guidance on due date calculation.
If your healthcare provider's ultrasound dating differs from this calculator by more than a week, your provider will likely revise the due date based on the scan. This is normal and does not indicate a problem with your pregnancy.
How Many Weeks Pregnant Am I?
Pregnancy is measured in gestational weeks, counted from the first day of your last menstrual period. This means at 'week 1' of pregnancy, you have not yet conceived — conception typically occurs around week 2. This can be confusing, but it is the standard used by healthcare providers worldwide.
Gestational age is different from embryonic or fetal age. An embryo at 8 gestational weeks is actually about 6 weeks old from the point of fertilization. When your doctor or midwife says 'you are 8 weeks pregnant,' they mean 8 weeks since your LMP, not 8 weeks since conception.
Pregnancy is divided into three trimesters: the first (weeks 1–12), the second (weeks 13–27), and the third (weeks 28–40). Each trimester brings different developmental milestones and different physical changes. The due date calculator above shows you exactly where you are in this journey based on your input.
Frequently Asked Questions
How is a pregnancy due date calculated?
The most common method is Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a standard 28-day cycle with ovulation on day 14. If your cycle length differs, the due date can be adjusted accordingly. Alternatively, the due date can be calculated from a known ovulation or conception date by adding 266 days (38 weeks), which represents the time from fertilization to delivery.
How accurate are due date calculators?
Due date calculators give a useful estimate, but only about 5% of babies are born on their exact due date. Most babies arrive within two weeks either side — between 38 and 42 weeks is considered the normal range. Ultrasound dating in the first trimester is more accurate than LMP-based calculations and is the standard used by healthcare providers when a discrepancy exists.
Can a due date change during pregnancy?
Yes. Due dates are often revised after an early ultrasound (typically performed at 8–12 weeks), which measures the baby's size to give a more accurate gestational age estimate. If the ultrasound date differs from the LMP-based date by more than a certain number of days, your healthcare provider may update your due date. Later ultrasounds are less accurate for dating purposes.
What week is considered full term?
Full term pregnancy is defined as 39–40 weeks of gestation. Babies born at 37–38 weeks are considered 'early term,' while those born at 41–42 weeks are 'late term.' The term 'full term' was redefined in 2013 to encourage avoiding elective deliveries before 39 weeks, when possible, as outcomes are generally better for babies born at 39 weeks or later.
When should I take a pregnancy test?
The most accurate time to take a home pregnancy test is on the first day of your missed period or later, which corresponds to approximately 14 days past ovulation (14 DPO). Testing earlier is possible with sensitive tests, but the risk of a false negative is higher. The ClearLine Pregnancy Test Calculator can help you find the best day to test based on your cycle.
