Maternal–fetal exchange during labor is diminished by:

Comprehensive and Detailed Explanation From NCC-Aligned Sources:

Placental calcifications:

Reduce surface area for maternal–fetal gas exchange

Impair placental perfusion

Are associated with post-dates and chronic insufficiency

Decrease the placenta’s ability to oxygenate the fetus

Why the incorrect answers are wrong:

A. Increased maternal cardiac output → improves uteroplacental perfusion.

B. Open-glottis pushing → improves oxygenation compared with closed-glottis Valsalva pushing.

Correct answer: Placental calcifications.