🤰 Gestational Age Calculator
Calculate Pregnancy Age & Estimated Due Date
LMP Method • Naegele's Rule • Accurate
📅 Calculate Gestational Age from LMP
Enter your last menstrual period (LMP) to calculate your current gestational age and estimated due date.
ℹ️ Last Menstrual Period (LMP): The first day of your last period, not the date of conception. Most pregnancies are dated from LMP.
🗓️ Calculate Estimated Due Date
Calculate your expected date of delivery using Naegele's Rule (LMP + 280 days).
ℹ️ Naegele's Rule: Add 9 months and 7 days to your LMP, or subtract 3 months and add 7 days, then add 1 year.
📊 Calculate Gestational Age from Due Date
If you know your estimated due date, calculate your current gestational age.
Understanding Gestational Age
Gestational age is the age of a pregnancy measured from the first day of the last menstrual period (LMP) to the current date. This measurement is used by healthcare providers to track pregnancy development, estimate the due date, and monitor fetal development.
⭐ Gestational Age vs Fetal Age: Gestational age is typically 2 weeks older than fetal age (chronological age since conception) because conception usually occurs about 2 weeks after the first day of the last menstrual period.
Gestational Age Calculation Formula
Gestational Age = (Current Date - LMP) ÷ 7 days = Weeks + Days
Example: If LMP was December 1, 2024, and today is January 15, 2025:
Days elapsed = 45 days, which equals 6 weeks and 3 days of gestation
Naegele's Rule for Estimated Due Date
Estimated Due Date = LMP + 280 days (40 weeks)
Alternative formula:
EDD = LMP - 3 months + 7 days + 1 year
For example, if LMP is December 1, 2024:
EDD = (December - 3 months = September) + 7 days + (2024 + 1 year = 2025) = September 8, 2025
Pregnancy Trimesters
| Trimester | Gestational Weeks | Duration | Key Developments |
|---|---|---|---|
| First Trimester | 0-13 weeks | ~3 months | Organ formation, heartbeat visible by 8-10 weeks, morning sickness common |
| Second Trimester | 14-27 weeks | ~3 months | Fetal movement, gender determination, rapid growth, morning sickness usually ends |
| Third Trimester | 28-42 weeks | ~3 months | Weight gain, position for delivery, prenatal visits increase, labor preparation |
Important Milestones by Gestational Age
- 6-8 weeks: Fetal heartbeat detectable by ultrasound
- 10-14 weeks: First trimester screening ultrasound (nuchal translucency)
- 18-22 weeks: Anatomy scan/mid-pregnancy ultrasound
- 20 weeks: Viability threshold (baby can potentially survive with medical support)
- 24 weeks: Age of viability standard (better survival rates)
- 28 weeks: Start of third trimester, fetal monitoring increases
- 35-37 weeks: Term pregnancy (safe for delivery)
- 37+ weeks: Full term pregnancy
- 40 weeks: Estimated due date (only ~5% deliver exactly on due date)
- 42 weeks: Post-term pregnancy (labor typically induced)
Methods to Calculate Gestational Age
- LMP Dating: Most common method, based on first day of last period
- Ultrasound Dating: Most accurate in first trimester, done at various intervals
- Physical Examination: Uterine size assessment (less accurate)
- Conception Date: Known date of intercourse or fertility treatment (Add 14 days to conception for LMP equivalent)
Accuracy of Gestational Age Estimation
| Dating Method | Timing Used | Accuracy |
|---|---|---|
| LMP | Entire pregnancy | ±3-5 days in first trimester, ±2-3 weeks by third trimester |
| First trimester ultrasound (11-14 weeks) | First 3 months | ±3-5 days |
| Second trimester ultrasound (15-20 weeks) | Middle of pregnancy | ±5-7 days |
| Third trimester ultrasound (after 20 weeks) | Final 3 months | ±2-3 weeks |
When to Seek Medical Attention
- Before 12 weeks: Vaginal bleeding, severe cramping, dizziness
- After 20 weeks: No fetal movement for extended period, vaginal bleeding, abdominal pain
- After 28 weeks: Reduced fetal movement, contractions, leaking fluid, vaginal bleeding
- After 37 weeks: Regular contractions, leaking fluid (signs of labor)
Frequently Asked Questions
Why is gestational age counted from LMP instead of conception?
The exact date of conception is usually unknown, whereas most women know or can find out the first day of their last menstrual period. Conception typically occurs about 2 weeks after LMP, so gestational age is roughly 2 weeks older than fetal age.
What if I don't remember my LMP?
Your healthcare provider can estimate gestational age through ultrasound. First trimester ultrasounds are most accurate (±3-5 days), while later ultrasounds are less precise. Provide any relevant dates like when you first felt movement or had a positive pregnancy test.
Is my due date exact?
No. The due date (EDD) is an estimate. Only about 5% of babies are born on their exact due date. Most babies are born within a week before or after the due date. A pregnancy is considered term between 37-42 weeks.
How accurate is Naegele's Rule?
Naegele's Rule is reasonably accurate for estimating due dates when LMP is known accurately. However, it assumes a standard 28-day cycle and 40-week pregnancy, which can vary. Early ultrasound dating is typically more accurate.
Can gestational age change during pregnancy?
Yes, if early ultrasound dating differs significantly from LMP dating, your due date may be adjusted. Ultrasounds are most accurate in early pregnancy, so adjustments usually occur early in pregnancy based on ultrasound findings.
What if I have an irregular menstrual cycle?
If your cycles are irregular, LMP dating may be less accurate. Your healthcare provider may rely more heavily on early ultrasound dating, which is more reliable in such cases.
When is the best time to have an ultrasound for dating?
The first trimester ultrasound (11-14 weeks) is most accurate for pregnancy dating (±3-5 days). The second trimester anatomy scan (18-22 weeks) is less accurate (±5-7 days), and third trimester ultrasounds are least accurate for dating (±2-3 weeks).
What does "40 weeks" of pregnancy really mean?
A standard pregnancy is calculated as 40 weeks from LMP. This equals approximately 266 days from conception (accounting for the 2-week difference between LMP dating and actual conception). Most full-term pregnancies go from 37-42 weeks.
Should I be concerned if my due date changes?
Minor adjustments (a few days) are common and usually not concerning. Larger adjustments may indicate differences in cycle length or ultrasound findings. Discuss any significant date changes with your healthcare provider.
Is there a difference between "due date" and "expected date of delivery"?
These terms are used interchangeably. Both refer to the estimated date of delivery (EDD), calculated using gestational age. It's an estimate, not a guarantee of when your baby will arrive.
Official Pregnancy & Gestational Age Sources
All pregnancy information sourced from official medical and healthcare organizations:
🏥 MDCalc - Pregnancy Due Dates Calculator 📊 OmniCalculator - Gestational Age Calculator 🤰 Perinatology - Due Date Calculator 👶 Tua Saúde - Gestational Age Guide 🏛️ Starship - Gestation Calculator ❤️ Healthify - Pregnancy Calculator