What can cause multiple gestation to result in higher AFP levels?

The correct response, which identifies that the presence of multiple fetuses can result in higher alpha-fetoprotein (AFP) levels, is rooted in the biology of fetal development and the production of AFP.

Alpha-fetoprotein is primarily produced by the developing fetal liver and is present in higher concentrations when there are multiple fetuses. In cases of multiple gestation (twins, triplets, etc.), each fetus contributes to the total AFP levels detected in the maternal serum. This means that with multiple fetuses, the aggregation of AFP production across all developing fetuses leads to significantly higher AFP levels when compared to a singleton pregnancy.

The physiological reasoning here is that each fetus has its own metabolic processes contributing to AFP production, and therefore, the total AFP level measured in the mother's bloodstream is elevated in multiple gestations due to the cumulative effect.

While factors such as maternal age, maternal weight, and fetal movements may influence the general maternal-fetal environment or other screening biomarkers, they do not directly relate to the specific enhancement of AFP production associated with the presence of multiple fetuses. Therefore, the increase in AFP levels is distinctly linked to the number of fetuses rather than these other factors.