1. Definition of Gestational Age
Gestational Age (GA):
- The number of completed weeks since the first day of the last menstrual period (LMP).
- Full-term pregnancy lasts approximately 280 days (40 weeks, 9 months).
Embryonic Age (Conceptional Age):
- Starts from fertilization (~2 weeks after LMP).
- Always 2 weeks less than gestational age.
2. Pregnancy Duration & Trimesters
|
Trimester |
Duration |
Key Events |
|
First Trimester |
0–12 weeks |
Organogenesis, Neural tube closure, Placental
formation |
|
Second Trimester |
13–27 weeks |
Fetal growth, Quickening (Fetal movements at ~18–20
weeks) |
|
Third Trimester |
28–40 weeks |
Maturation of organs, Fetal weight gain, Preparation
for birth |
- Term Pregnancy: 37–42 weeks
- Preterm Pregnancy: <37 weeks
- Post-term Pregnancy: >42 weeks
3. Methods of Gestational Age Estimation
1. Last Menstrual Period (LMP) Method
- Naegele’s Rule for Estimated Due Date (EDD):
- EDD = LMP + 9 months + 7 days
- Example: If LMP = June 10, EDD = March 17.
2. Ultrasound Dating (Most Accurate Method)
- First Trimester (Best Accuracy):
- Crown-Rump Length (CRL) at 7–13 weeks → ±5 days accuracy.
- Second Trimester:
- Biparietal Diameter (BPD), Femur Length (FL), Head Circumference (HC).
- Third Trimester:
- Less accurate due to fetal growth variations.
3. Fundal Height Measurement
- At 12 weeks: Fundus at pubic symphysis.
- At 20 weeks: Fundus at umbilicus.
- After 20 weeks: Fundal height (cm) ≈ Gestational age (weeks).
4. Clinical Importance of Gestational Age
1. Preterm vs. Term Birth Risks
- Preterm (<37 weeks): Respiratory Distress Syndrome, NEC, IVH.
- Post-term (>42 weeks): Placental insufficiency, Meconium aspiration.
2. Timing of Screening Tests
- First Trimester Screening (11–14 weeks): Nuchal Translucency, PAPP-A, hCG.
- Anatomy Scan (18–22 weeks): Congenital anomalies, Fetal growth.
- Glucose Screening (24–28 weeks): Gestational Diabetes Test.
Summary Table: Gestational Age & Pregnancy Duration
|
Parameter |
Value |
|
Full-Term Pregnancy |
37–42 weeks |
|
EDD Calculation (Naegele’s
Rule) |
LMP + 9 months + 7 days |
|
Best Ultrasound Dating |
First Trimester (CRL) |
|
Fundal Height at 20 Weeks |
At the umbilicus |
|
Preterm vs. Post-term Risks |
RDS, NEC vs. Placental Insufficiency |
Key Takeaways
- Gestational age is calculated from LMP, but ultrasound is most accurate.
- First trimester ultrasound (CRL) is the best method for dating pregnancy.
- Fundal height correlates with gestational age after 20 weeks.
- Preterm (<37 weeks) and post-term (>42 weeks) pregnancies require close monitoring.
