Which of the following triggers the closure of the foramen ovale at birth?

A. Clamping of the umbilical cord eliminates the low-resistance placental circulation, which previously accounted for a significant portion of fetal blood flow. This increases systemic vascular resistance as blood is now redirected through the neonatal systemic circulation.

B. The first breath and lung expansion lead to a dramatic decrease in pulmonary vascular resistance, which increases left atrial pressure. This shift in pressure gradients contributes to the closure of fetal shunts and an increase in systemic vascular resistance.

D. Closure of the foramen ovale occurs due to increased left atrial pressure following lung expansion and decreased right atrial pressure after umbilical cord clamping. This closure redirects blood flow through the systemic circulation, contributing to the rise in systemic vascular resistance.

Rationale for incorrect answers

C. Prostaglandin E2 levels decrease at birth, not increase. This decline facilitates the closure of the ductus arteriosus and other fetal shunts. Increased prostaglandin E2 would maintain patency of these shunts, opposing the rise in systemic vascular resistance.

E. Activation of the renal system is not an immediate factor in the increase in systemic vascular resistance at birth. While renal function begins to regulate fluid and electrolyte balance postnatally, it does not directly influence the acute changes in vascular resistance during the transition to neonatal circulation.

Take home points

• Clamping the umbilical cord eliminates the low-resistance placental circuit, increasing SVR.
• Lung expansion reduces pulmonary vascular resistance and redirects blood flow.
• Closure of fetal shunts, including the foramen ovale, contributes to increased SVR.
• Prostaglandin E2 levels decrease at birth, facilitating shunt closure.

Non-stress test interpretation evaluates fetal heart rate patterns in response to movement, primarily assessing oxygenation, autonomic integrity, placental function, and neurologic status. In term fetuses, normal baseline fetal heart rate ranges from 110–160 bpm with variability of 6–25 bpm. A reactive NST shows accelerations of ≥15 bpm lasting ≥15 seconds, suggesting intact sympathetic and parasympathetic regulation and adequate fetal oxygenation.

Rationale for correct answers

B. A reactive non-stress test reflects fetal well-being. It indicates sufficient oxygen delivery via the placenta and intact autonomic response. The presence of at least 2 accelerations within a 20-minute window in term fetuses confirms reactivity and a reassuring status.

Rationale for incorrect answers

A. Fetal distress is typically associated with absent accelerations, decreased variability, or late decelerations. A reactive NST, by definition, rules out distress at the time of testing and suggests normal fetal acid–base balance.

C. Uteroplacental insufficiency manifests as absent accelerations, decreased variability, or late decelerations. These would lead to a non-reactive NST, not a reactive one, which implies that placental perfusion is adequate.

D. Cord compression leads to variable decelerations, not accelerations. If present, these would be noted during monitoring and may signal concern depending on frequency and severity. A reactive NST lacks these deceleration patterns.

Take home points

• Reactive NST suggests adequate fetal oxygenation and neurologic status.
• Requires 2 accelerations of ≥15 bpm lasting ≥15 seconds in 20 minutes.
• Non-reactive NST may signal placental or umbilical pathology.