Understanding Pregnancy Dating
How is Due Date Calculated?
The medical standard uses 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.
Due Date = LMP + 1 year - 3 months + 7 days
Example: LMP = Jan 1, 2024 → Due Date = Oct 8, 2024
LMP vs. Conception Date Methods
- LMP Method (Standard): Counts from first day of last period. Used by doctors because most women know this date. Pregnancy is dated as starting 2 weeks before actual conception.
- Conception Date Method: Counts from actual (or estimated) day of conception. Adds 266 days (38 weeks) instead of 280. More accurate if you know exact conception date (IVF, ovulation tracking).
The Three Trimesters
First Trimester (Weeks 1-12)
- Critical period of organ development (organogenesis)
- Common symptoms: morning sickness, fatigue, breast tenderness
- First ultrasound (6-9 weeks) to confirm pregnancy and due date
- Miscarriage risk highest in this trimester
- Taking folic acid is crucial for neural tube development
Second Trimester (Weeks 13-26)
- Often called the "golden period" - energy returns, morning sickness subsides
- Baby movements felt (quickening) around weeks 18-22
- Anatomy scan (weeks 18-22) checks development and can reveal gender
- Visible baby bump develops
- Glucose screening test (weeks 24-28) for gestational diabetes
Third Trimester (Weeks 27-40+)
- Rapid weight gain as baby grows to full size
- More frequent doctor visits (every 2 weeks, then weekly after 36 weeks)
- Braxton Hicks contractions (practice contractions)
- Preparing for birth: nesting instinct, childbirth classes, hospital bag
- Baby drops lower into pelvis (lightening) as due date approaches
Important Pregnancy Milestones
⚠️ Important Note
This calculator provides estimates based on standard calculations. Your healthcare provider may adjust your due date based on ultrasound measurements, especially from the first trimester, which are more accurate for dating. Always consult your doctor for personalized prenatal care.
Why Only 5% of Babies Arrive on Due Date?
A due date is an estimate, not a deadline. Natural variation in pregnancy length, differences in cycle length, timing of ovulation, and individual factors mean most babies arrive within a 5-week window (37-42 weeks). First-time mothers often go past their due date, while subsequent pregnancies may deliver earlier.