Pregnancy Due Date Calculator
Pregnancy Timeline
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Understanding Pregnancy Due Dates
How Due Dates Are Calculated
The estimated due date (EDD) is calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP). This is known as Naegele's rule. However, only about 5% of babies are born on their exact due date.
Naegele's Rule
Due Date = LMP + 280 days (40 weeks)
Or: LMP - 3 months + 7 days + 1 year
Understanding Gestational Age
Pregnancy is measured from the first day of your last period, not from conception. This means when you're "4 weeks pregnant," your baby has only been developing for about 2 weeks. Ovulation and conception typically occur around day 14 of your cycle.
Gestational Age
Counted from LMP. Used for all medical purposes and developmental milestones. Your "week 8" ultrasound is at 8 weeks gestational age.
Fetal Age
Actual time since conception. About 2 weeks less than gestational age. Less commonly used but reflects true development time.
Pregnancy Trimesters
First Trimester (Weeks 1-12)
Major organ development. Morning sickness common. Risk of miscarriage highest. Baby grows from a single cell to about 3 inches long.
Second Trimester (Weeks 13-26)
Often called the "golden period." Energy returns, morning sickness fades. You'll feel baby move. Anatomy scan around week 20.
Third Trimester (Weeks 27-40)
Baby gains weight rapidly. You may feel more uncomfortable. Braxton Hicks contractions. Final preparations for birth.
Key Milestones
| Week | Development |
|---|---|
| 5-6 | Heartbeat may be detected |
| 12 | All major organs formed; risk of miscarriage drops |
| 16-20 | Gender visible; you may feel movement (quickening) |
| 24 | Viability milestone - baby could survive outside womb |
| 37 | Full term begins - baby is ready for birth |
| 40 | Due date |
When Is It Safe to Deliver?
Important Note
Due dates are estimates, not exact predictions. A normal pregnancy can range from 37 to 42 weeks. Your healthcare provider may adjust your due date based on early ultrasound measurements, which are more accurate than LMP-based calculations for women with irregular cycles.
Frequently Asked Questions
How accurate is a due date calculator?
Due date calculators provide an estimate, but only about 5% of babies are born on their exact due date. Most babies are born within 2 weeks before or after the estimated date. Early ultrasound dating (before 12 weeks) is more accurate than LMP-based calculations, especially for women with irregular cycles.
How is the pregnancy due date calculated?
The due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Alternatively, add 266 days to your conception date if known. This assumes a 28-day cycle with ovulation on day 14. Your healthcare provider may adjust based on ultrasound measurements.
What are the three trimesters of pregnancy?
The first trimester spans weeks 1-12 and involves major organ development. The second trimester (weeks 13-26) is often called the 'golden period' when energy returns and you feel baby movement. The third trimester (weeks 27-40) involves rapid fetal weight gain and preparation for birth.
When is a baby considered full term?
A baby is considered full term at 39-40 weeks. Early term is 37-38 weeks, late term is 41 weeks, and post-term is 42+ weeks. While babies born at 37 weeks can be healthy, waiting until 39 weeks allows important brain and lung development. Doctors typically don't let pregnancies continue past 42 weeks.