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Understanding Pregnancy Dates
How Due Dates Are Calculated
Pregnancy is typically measured from the first day of your last menstrual period (LMP), not from conception. This is because ovulation and conception dates are often uncertain, while the LMP date is usually known.
Naegele's Rule
The most common method: Add 280 days (40 weeks) to the first day of your LMP, or add 7 days to LMP then subtract 3 months. This assumes a 28-day cycle with ovulation on day 14.
The Three Trimesters
First Trimester
Weeks 1-12
- • Major organ development
- • Morning sickness common
- • Heartbeat detectable (6-7 weeks)
- • Highest miscarriage risk
Second Trimester
Weeks 13-26
- • Baby bump visible
- • Feel baby movement
- • Gender can be determined
- • Often called "honeymoon phase"
Third Trimester
Weeks 27-40
- • Rapid weight gain
- • Baby fully developed by 37 weeks
- • Increased discomfort
- • Preparing for delivery
Key Pregnancy Milestones
| Week | Development | Size Comparison |
|---|---|---|
| 4 | Implantation, positive pregnancy test possible | Poppy seed |
| 8 | All major organs forming, limbs developing | Raspberry |
| 12 | First trimester complete, reflexes developing | Lime |
| 16 | Facial features formed, may feel movement | Avocado |
| 20 | Halfway point, anatomy scan, gender visible | Banana |
| 24 | Viability milestone, hearing developed | Corn |
| 32 | Practicing breathing, bones hardening | Squash |
| 40 | Full term, ready for birth | Small pumpkin |
Due Date Accuracy
Only 5% Deliver on Due Date
Due dates are estimates. Only about 5% of babies are born on their exact due date. Most births occur within 2 weeks before or after the due date. First-time mothers often deliver a bit later than their due date.
Terms and Definitions
Gestational Age
Age of pregnancy counted from LMP. A full-term pregnancy is about 40 weeks gestational age.
Fetal Age
Age from conception, about 2 weeks less than gestational age. Also called "conceptional age."
LMP
Last Menstrual Period - the first day of your last period before pregnancy.
EDD
Estimated Due Date (or Expected Date of Delivery) - the projected birth date.
Frequently Asked Questions
How is my pregnancy due date calculated?
Due dates are calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. Early ultrasounds (before 12 weeks) can provide more accurate dating, especially for women with irregular cycles.
What are the three trimesters of pregnancy?
The first trimester (weeks 1-12) involves major organ development and carries the highest miscarriage risk. The second trimester (weeks 13-26) is often called the 'honeymoon phase' when energy returns. The third trimester (weeks 27-40) involves rapid baby growth and preparation for birth.
How accurate is my estimated due date?
Only about 5% of babies are born on their exact due date. Most births occur within 2 weeks before or after the due date. First-time mothers often deliver a bit later. Full term is now defined as 39-40 weeks, with babies born at 37-38 weeks considered 'early term.'
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last menstrual period and is used for all medical purposes - a full-term pregnancy is 40 weeks gestational age. Fetal age (embryonic age) counts from conception and is about 2 weeks shorter. When your doctor says you're 10 weeks pregnant, the baby has been developing for about 8 weeks.