**Core Concept**
The World Health Organization (WHO) defines preterm birth as the birth of a baby before 37 weeks of completed gestation. This definition is crucial in understanding the risk factors, complications, and outcomes associated with preterm birth.

**Why the Correct Answer is Right**
Preterm birth is a significant cause of morbidity and mortality in newborns. Babies born before 37 weeks of gestation are at increased risk of respiratory distress syndrome, intraventricular hemorrhage, and necrotizing enterocolitis, among other complications. The earlier the gestation, the higher the risk of these complications. The 37-week cutoff is a critical threshold because the lungs and other vital organs of the fetus are not fully developed before this gestational age.

**Why Each Wrong Option is Incorrect**
**Option B:** Although the American College of Obstetricians and Gynecologists (ACOG) suggests that babies born between 37 weeks and 38 weeks and 6 days of gestation may be at higher risk for complications, the WHO definition specifically defines preterm birth as before 37 weeks.

**Option C:** The 39-week gestation is often considered a threshold for full-term birth, but it is not the definition of preterm birth.

**Option D:** Babies born before 40 weeks of gestation may still be at risk for complications, but the definition of preterm birth specifically refers to births before 37 weeks.

**Clinical Pearl / High-Yield Fact**
To remember the definition of preterm birth, use the mnemonic "37 is the key": 37 weeks of gestation is the critical threshold beyond which babies are considered full-term.

**Correct Answer:**
✓ Correct Answer: A. Bom before 37 weeks of gestations