Gestational Age Explained
Gestational age is the most common way to measure pregnancy. It is counted from the first day of your last menstrual period (LMP), not from the date of conception. This guide explains why, how it is calculated, and what it means for your due date.
What Is Gestational Age?
Gestational age is the age of the pregnancy counted from the first day of the last menstrual period (LMP). It is expressed in weeks and days. For example, "22 weeks 3 days" is written as 22+3.
This is the standard method used by healthcare providers and recommended by ACOG. It is used to:
- Calculate the estimated due date
- Determine the trimester
- Schedule screening tests and ultrasounds
- Assess fetal growth and development
- Classify term (preterm, early term, full term, etc.)
Why Count from the Last Period?
Pregnancy is counted from the LMP because most people know the date of their last period, but very few know the exact date of conception.
This means:
- At the moment of conception, you are already considered 2 weeks pregnant.
- At the time of a missed period, you are considered 4 weeks pregnant.
- Your due date is 40 weeks from your LMP, which is about 38 weeks from conception.
How Gestational Age Is Calculated
Gestational age = Reference date − (LMP + cycle adjustment)
The cycle adjustment accounts for cycles that are not exactly 28 days:
Cycle-corrected LMP = LMP + (cycle length − 28)
For example, if your LMP was January 1 and your cycle is 32 days:
- Cycle adjustment = 32 − 28 = 4 days
- Cycle-corrected LMP = January 1 + 4 days = January 5
- Estimated due date = January 5 + 280 days = October 12
Gestational Age vs. Fetal Age
Gestational age and fetal age are different:
| Measurement | Counted from | At conception | At missed period | At due date |
|---|---|---|---|---|
| Gestational age | LMP | 2 weeks | 4 weeks | 40 weeks |
| Fetal age | Conception | 0 weeks | 2 weeks | 38 weeks |
Healthcare providers use gestational age. When someone says "I am 20 weeks pregnant," they mean gestational age.
How Accurate Is Gestational Age?
LMP-based dating is reasonably accurate if:
- Your cycles are regular
- You know the date of your last period
- You are not currently breastfeeding or have recently stopped hormonal contraception
A first-trimester ultrasound is more accurate than LMP-based dating. If the two differ by more than 7 days, ACOG recommends using the ultrasound date.
Ultrasound Dating Accuracy by Trimester
| Trimester | Accuracy | Method |
|---|---|---|
| First | ± 5 days | Crown-rump length (CRL) |
| Second | ± 8-14 days | Biparietal diameter, femur length |
| Third | ± 21-28 days | Too unreliable to redate |
Source: ACOG Committee Opinion 700, Methods for Estimating the Due Date.
When to Talk to Your Clinician
Contact your healthcare provider if:
- You are unsure of your LMP date
- Your cycles are irregular
- You have questions about your gestational age or due date
- Your ultrasound dating differs from your LMP dating