The diameter of fetal head, which comes in totally deflexed head :
**Core Concept**
The diameter of the fetal head in a totally deflexed position refers to the measurement of the head's widest part, which is essential for obstetricians to assess during delivery. This concept is crucial in determining the safest method of vaginal delivery and identifying potential complications.
**Why the Correct Answer is Right**
In a totally deflexed head, the occipital bone is in contact with the sacrum, and the head is in a position where the widest part is the mentovertical diameter. This measurement is crucial as it determines the suitability of vaginal delivery and the need for instrumental delivery or cesarean section. The mentovertical diameter is the distance from the chin (menton) to the highest point of the vertex (vertex) of the skull. This measurement is typically around 13-14 cm in a normal fetus.
**Why Each Wrong Option is Incorrect**
**Option A:** The biparietal diameter (BPD) is the measurement between the two parietal bones and is not relevant in a totally deflexed head.
**Option B:** The occipitofrontal diameter is the measurement from the occipital bone to the frontal bone and is not the widest part of the head in a deflexed position.
**Option C:** The suboccipitobregmatic diameter is the measurement from the suboccipital region to the bregma and is relevant in a flexed head, not a deflexed head.
**Clinical Pearl / High-Yield Fact**
In obstetrics, the position and size of the fetal head are critical in determining the mode of delivery. Remember that a totally deflexed head with a mentovertical diameter of 13-14 cm is generally suitable for vaginal delivery.
**Correct Answer:** C.