Due Date Calculator - Free Online Pregnancy Calculator
Due Date Inputs
The first day of your last menstrual period
About Due Date Calculation
Naegele's Rule: adds 280 days (40 weeks) to your LMP date.
Enter your information and click "Calculate Due Date" to see your pregnancy timeline, checkup schedule, and important milestones
Understanding Due Date Calculation
Your estimated due date (EDD) is the date your healthcare provider expects your baby to be born, calculated as 40 weeks (280 days) from the first day of your last menstrual period (LMP). This calculation method, called Naegele's Rule, has been used since the 1830s.
It's important to understand that a due date is an estimate, not a guarantee. Only about 4% of babies arrive on their exact due date. Most babies (about 90%) are born within two weeks before or after the due date. This is why doctors refer to a "due date range" rather than a specific day.
Why 40 Weeks?
The 40-week pregnancy timeline is measured from your LMP, not from conception. Since conception typically occurs about 2 weeks after your LMP (during ovulation), the actual fetal development time is approximately 38 weeks. This two-week difference exists because it's easier to identify the start of menstruation than the exact moment of conception.
Due Date Calculation Methods
📅 Last Menstrual Period (LMP)
Most Common Method
- • Formula: LMP + 280 days
- • Assumes 28-day cycle
- • Ovulation on day 14
- • Used for initial estimation
🎯 Conception Date
If You Know Conception
- • Formula: Conception + 266 days
- • More accurate if known
- • Common with IVF/IUI
- • 38-week fetal age
🔬 Ultrasound Dating
Most Accurate Method
- • Crown-rump length (CRL)
- • Accurate within 3-5 days
- • Best at 11-14 weeks
- • Overrides LMP if ≥7 days off
Pregnancy Trimesters
First Trimester (Weeks 1-13)
Baby Development:
- All major organs form
- Heart begins beating (week 5-6)
- Limbs develop
- Brain and spinal cord form
Mom's Experience:
- Morning sickness common
- Fatigue and breast tenderness
- Frequent urination
- Emotional changes
Second Trimester (Weeks 14-26)
Baby Development:
- Gender visible on ultrasound
- Hair and nails grow
- Baby can hear sounds
- Movements felt (quickening)
Mom's Experience:
- Energy returns ("honeymoon phase")
- Baby bump becomes visible
- Reduced nausea
- Feel baby movements
Third Trimester (Weeks 27-40)
Baby Development:
- Rapid weight gain
- Lungs mature
- Baby practices breathing
- Positioned for birth
Mom's Experience:
- Increased discomfort
- Braxton Hicks contractions
- Shortness of breath
- Preparing for labor
Frequently Asked Questions
How is due date calculated from last menstrual period?
Due date is calculated using Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day menstrual cycle with ovulation occurring on day 14. The formula: Due Date = LMP + 280 days. For example, if your LMP was January 1, your due date would be October 8. This method has been used since the 1830s and remains the standard initial dating method. However, ultrasound dating between 11-14 weeks is considered more accurate and may adjust your due date by several days.
Which due date calculation method is most accurate?
Ultrasound dating performed between 11-14 weeks of pregnancy is the most accurate method, with accuracy within 3-5 days. Early ultrasound measures crown-rump length (CRL) of the fetus, which correlates strongly with gestational age. LMP-based calculation is accurate if you have regular 28-day cycles and know your exact LMP date, but can be off by 1-2 weeks with irregular cycles. Conception date method is accurate only if you know the exact date of conception. Medical guidelines recommend using the earliest ultrasound for dating when there's a discrepancy of more than 7 days between LMP and ultrasound dating.
What is full term pregnancy and when is it safe to deliver?
Full term pregnancy is defined by the American College of Obstetricians and Gynecologists (ACOG) as 37-42 weeks. Specifically: Early term is 37 weeks 0 days to 38 weeks 6 days. Full term is 39 weeks 0 days to 40 weeks 6 days. Late term is 41 weeks 0 days to 41 weeks 6 days. Post term is 42 weeks 0 days onward. Babies born at 39-40 weeks have the lowest risk of complications. Delivery before 37 weeks is considered preterm and carries higher risks. After 42 weeks, risks increase and induction is typically recommended. Only 4% of babies arrive on their exact due date, with most arriving within 2 weeks before or after.
How many prenatal checkups should I have during pregnancy?
Standard prenatal care schedule includes 10-15 visits for uncomplicated pregnancies. Schedule: First visit at 8-12 weeks for dating ultrasound and initial screening. Monthly visits from weeks 4-28 (approximately 6 visits). Bi-weekly visits from weeks 28-36 (approximately 4 visits). Weekly visits from weeks 36-40 (approximately 4-5 visits). Key appointments include: 8-12 weeks (dating ultrasound), 16 weeks (maternal serum screening), 18-22 weeks (anatomy scan), 24-28 weeks (glucose screening), 36 weeks (Group B strep test). High-risk pregnancies require more frequent monitoring. Virtual visits may supplement in-person appointments.
What are the three trimesters of pregnancy?
Pregnancy is divided into three trimesters based on fetal development stages. First Trimester (Weeks 1-13): Major organs form, morning sickness common, miscarriage risk highest, fetal heartbeat detectable at 6 weeks. Second Trimester (Weeks 14-26): Often called "honeymoon phase" with increased energy, baby movements felt (18-25 weeks), anatomy scan at 20 weeks, gender typically visible. Third Trimester (Weeks 27-40): Rapid fetal growth, increased discomfort, Braxton Hicks contractions, baby drops into pelvis, final preparations for birth. Each trimester brings unique physical changes, fetal developments, and medical screenings. Understanding trimester milestones helps expectant parents know what to expect.
External Resources & Further Reading
Learn more about pregnancy, prenatal care, and baby development from these trusted medical authorities:
ACOG - Fetal Growth & Development
American College of Obstetricians and Gynecologists guidelines on pregnancy timeline and fetal development milestones.
acog.org →Mayo Clinic - Prenatal Care
Comprehensive prenatal care guide, checkup schedules, and what to expect during pregnancy from Mayo Clinic experts.
mayoclinic.org →CDC - Pregnancy Information
Centers for Disease Control pregnancy health guidelines, safety information, and recommended screenings.
cdc.gov →NIH - Pregnancy Research
National Institutes of Health research on pregnancy health, fetal development, and maternal well-being.
nichd.nih.gov →