Pregnancy Due Date Calculator
Calculate your estimated due date based on the first day of your last menstrual period. Get personalized insights about your pregnancy timeline, gestational age, and developmental milestones.
How Due Dates Are Calculated
This calculator uses Naegele's rule, the standard method used by healthcare providers. It adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). The calculation assumes a 28-day cycle with ovulation occurring on day 14 - if your cycle is different, the result is adjusted accordingly.
How to Use This Calculator
- Enter the first day of your last menstrual period (LMP)
- Adjust your average cycle length if it differs from 28 days
- Click "Calculate" to see your estimated due date
- Review your gestational age, trimester, and developmental insights
Frequently Asked Questions
Only about 5% of babies are born on their exact due date. The estimated due date is the midpoint of a range - most full-term babies are born between 37 and 42 weeks. First-trimester ultrasound is the most accurate method for dating pregnancy, especially if you have irregular cycles or are unsure of your LMP.
Ultrasound dating measures the baby's size and compares it to average growth. If your cycle is irregular, if ovulation occurred earlier or later than expected, or if you're unsure of your LMP, ultrasound may give a different date. First-trimester ultrasounds (before 14 weeks) are accurate to within 5-7 days. Your provider may adjust your due date based on ultrasound findings.
If you don't remember your last period or have irregular cycles, an early ultrasound is the best way to date your pregnancy. Your healthcare provider can measure the embryo or fetus to estimate gestational age accurately. This is particularly important for women with polycystic ovary syndrome (PCOS) or those who recently stopped hormonal birth control.
Gestational age is counted from the first day of your last menstrual period (LMP), which is about 2 weeks before conception. Fetal age (or conceptional age) is counted from actual fertilization. Healthcare providers use gestational age because it's easier to track - conception date is rarely known precisely. So when you're "6 weeks pregnant," the embryo is actually about 4 weeks old.
Pregnancy is divided into three trimesters:
- First Trimester: Weeks 1-13 (major organ formation)
- Second Trimester: Weeks 14-27 (rapid growth, movement felt)
- Third Trimester: Weeks 28-40 (final development, preparation for birth)
Going past your due date is common - about 30% of pregnancies go past 40 weeks. Your provider will monitor you more closely with non-stress tests and ultrasounds to check amniotic fluid levels and baby's well-being. Most providers recommend induction between 41-42 weeks, as risks increase after this point. Always discuss your options with your healthcare provider.
Calculator Limitations
- LMP-based calculation: Assumes you know your exact LMP date and have regular cycles
- Ovulation timing: Assumes ovulation occurs 14 days before your next period, which varies between women
- Cycle length: Accuracy decreases with very short (<21 days) or very long (>35 days) cycles
- Not a substitute for ultrasound: First-trimester ultrasound provides more accurate dating
- Due date is an estimate: Normal delivery range is 37-42 weeks
Medical Disclaimer: This calculator provides an estimate only and is not a substitute for prenatal care. Always confirm your due date with your healthcare provider, who may adjust it based on ultrasound findings. Schedule prenatal appointments as recommended by your provider.
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