Pregnancy Due Date Calculator
Estimate your due date and current pregnancy week from your last period.
An estimate (Naegele's rule). Only a doctor or ultrasound can confirm your due date.
How the Pregnancy Due Date Calculator Works
This calculator estimates your estimated due date (EDD) using the most widely used clinical method: Naegele's rule. It counts forward from the first day of your last menstrual period (LMP) to project when a full-term pregnancy of about 40 weeks would conclude.
The underlying logic assumes a regular 28-day menstrual cycle with ovulation around day 14. Because pregnancy is conventionally dated from the LMP rather than from conception, the 40-week (280-day) count includes roughly two weeks before you actually conceived. This is why a due date is sometimes described as 38 weeks from conception but 40 weeks from your last period.
Enter the first day of your last period and the calculator returns your projected due date along with your current gestational age in weeks and days. Remember that this is a projection: only about 1 in 20 babies arrive on the exact estimated date, and a full-term delivery anywhere from 37 to 42 weeks is considered normal.
The Formula: Naegele's Rule
Naegele's rule is a simple, time-tested calculation:
Due date = First day of LMP + 280 days (40 weeks)
A common shorthand version of the same rule is: take the first day of your last period, subtract three months, and add seven days (and one year). Both approaches arrive at essentially the same result.
Your current gestational week is calculated as the number of days between your LMP and today, divided by seven. For example, if 70 days have passed since the first day of your last period, you are 10 weeks pregnant (70 ÷ 7 = 10).
If your cycles are consistently longer or shorter than 28 days, the LMP-based estimate may be off by several days, because ovulation would have occurred earlier or later than day 14. In those cases, an early ultrasound is usually more accurate.
A Worked Example
Suppose the first day of your last menstrual period was January 1.
- Add 280 days: this lands on approximately October 8 of the same year.
- Using the shorthand rule: January minus three months is October; January 1 plus seven days is October 8.
If today were March 12, the time elapsed since January 1 would be about 70 days, placing you at roughly 10 weeks of gestation. The calculator handles these date computations automatically, including leap years and month-length differences.
Keep in mind that if you know your conception date precisely (for example, through assisted reproduction or careful cycle tracking), your provider may date the pregnancy from that point instead, which can shift the estimate slightly.
Why It Is Only an Estimate
An estimated due date is a planning tool, not a guarantee. Several factors influence when labor actually begins, including whether this is your first pregnancy, your cycle length, and individual biological variation. First-time pregnancies, on average, run slightly past the estimated date.
Healthcare providers often refine the due date with a first-trimester ultrasound, which measures the embryo and is generally the most reliable dating method early in pregnancy. If an ultrasound estimate differs substantially from the LMP estimate, clinicians typically adjust the due date accordingly.
Use this calculator to get a helpful sense of your timeline, but always confirm your due date and prenatal care plan with a qualified healthcare professional.
Frequently Asked Questions
For people with regular 28-day cycles, the LMP method is reasonably accurate, but only about 5% of babies are born on the exact estimated date. A first-trimester ultrasound is usually more precise, especially if your cycles are irregular. Your provider may adjust the date based on ultrasound findings.
If you are unsure of your LMP, an early ultrasound can date the pregnancy by measuring the embryo. This is often more reliable than an uncertain LMP. Speak with your healthcare provider, who can arrange dating scans and confirm your gestational age.
Yes. Naegele's rule assumes a 28-day cycle with ovulation around day 14. If your cycle is consistently longer or shorter, ovulation and conception shift accordingly, which can move your true due date by several days. An ultrasound helps account for this variation.
Gestational age is counted from the first day of your last period and is the standard clinical measure, totaling about 40 weeks. Conception age is counted from fertilization and is roughly two weeks shorter. Most pregnancy milestones and due dates use gestational age.
Yes. A delivery anywhere between 37 and 42 weeks is considered full term and normal. Babies born before 37 weeks are preterm, and pregnancies continuing past 42 weeks are monitored closely. Your provider will guide care based on your individual situation.
Use the estimated due date as a general guide for planning, but stay flexible, since the actual date can vary by weeks. For medical decisions, prenatal scheduling, and any concerns about your pregnancy, always consult your healthcare provider.