Commonest cause of non-engaged head in primi at term is
**Core Concept**
The question is testing the understanding of the obstetric concept of "non-engaged head" in primigravidas (first-time mothers) at term, which is a condition where the fetal head does not descend into the pelvis. This condition can be caused by various factors, including uterine anomalies, fetal malposition, or pelvimetry issues.
**Why the Correct Answer is Right**
The correct answer is related to the concept of pelvimetry, which is the measurement of the pelvic dimensions to assess the ability of the pelvis to accommodate the fetal head during labor. In primigravidas, the pelvis is often not well-prepared to accommodate the larger fetal head, leading to non-engagement. Specifically, the **Option A: Bicornuate uterus** is a type of uterine anomaly where the uterus is partially divided, leading to a smaller pelvic outlet and a higher risk of non-engagement of the fetal head.
**Why Each Wrong Option is Incorrect**
* **Option B:** Anencephaly is a congenital anomaly where the fetus lacks a significant portion of the brain and skull, which would not directly cause non-engagement of the head.
* **Option C:** Fetal macrosomia refers to a condition where the fetus is significantly larger than average, which can increase the risk of non-engagement, but it is not the most common cause in primigravidas at term.
**Clinical Pearl / High-Yield Fact**
In obstetrics, the term "primigravidas" refers to first-time mothers, and "multigravidas" refers to women who have been pregnant before. Understanding the pelvic dimensions and their relation to fetal head engagement is crucial in obstetric management.
**Correct Answer: A. Bicornuate uterus**