Free Due Date Calculator
Calculate your pregnancy due date from your last menstrual period. Shows current week, trimester, and days remaining instantly.
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Formula
Due Date = LMP + 280 days (Naegele's Rule) — adds 40 weeks from the first day of your last menstrual periodHow Your Pregnancy Due Date Is Calculated
This due date calculator uses Naegele's Rule, the same method used by doctors and midwives worldwide. It works by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). While conception typically occurs around day 14 of your cycle, pregnancy is dated from the LMP for consistency.
Keep in mind that your due date is an estimate, not a guarantee. Only about 5% of babies arrive on the exact predicted date. Most healthy pregnancies result in delivery between 37 and 42 weeks.
Understanding Your Results
Estimated Due Date is the date 280 days from your LMP. This is the date your healthcare provider will use for planning purposes.
Current Week shows how far along you are in weeks and days. This is important because many prenatal tests and milestones are scheduled by gestational week.
Trimester tells you which phase of pregnancy you're in. Each trimester lasts roughly 13 weeks and brings different physical changes and fetal developments.
Key Pregnancy Milestones by Week
- Week 6-8: First heartbeat detectable on ultrasound
- Week 12: End of first trimester; risk of miscarriage drops significantly
- Week 18-22: Anatomy scan ultrasound; many parents learn the sex
- Week 24: Viability milestone — baby could survive outside the womb with intensive care
- Week 28: Third trimester begins; glucose screening test
- Week 37: Considered early term; baby's lungs are typically mature
- Week 39-40: Full term; ideal window for delivery
Factors That Affect Your Due Date
Several factors can shift your actual delivery date from the calculated estimate:
- Irregular cycles: If your cycle isn't 28 days, ovulation timing varies
- First pregnancy: First-time mothers tend to deliver slightly later on average
- Multiple pregnancies: Twins and multiples often arrive 2-4 weeks early
- Maternal age and health: Various health conditions can affect timing
Your healthcare provider may adjust your due date based on ultrasound measurements, especially if done in the first trimester when dating is most accurate.
Frequently Asked Questions
How accurate is a due date calculator?
Due date calculators based on LMP are accurate to within about 2 weeks. Only about 5% of babies are born on their exact due date. Most arrive within a window of 37 to 42 weeks, which is considered full term.
What is Naegele's Rule?
Naegele's Rule is the standard method doctors use to estimate due dates. It adds 280 days (40 weeks) to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation occurring on day 14.
What if my cycle is longer or shorter than 28 days?
If your cycle is consistently longer or shorter than 28 days, your due date may shift. For example, if you have a 35-day cycle, ovulation likely occurs around day 21 instead of day 14, so your due date would be about a week later than the standard calculation.
When do the trimesters start and end?
The first trimester spans weeks 1-12, the second trimester covers weeks 13-26, and the third trimester runs from week 27 until delivery (typically around week 40). Each trimester brings different developmental milestones and symptoms.
Can an ultrasound change my due date?
Yes. A first-trimester ultrasound (before 13 weeks) is the most accurate way to date a pregnancy. If the ultrasound date differs from your LMP-based date by more than 7 days, your doctor will likely adjust the due date to match the ultrasound measurement.