**Core Concept**
Nysten's rule is a clinical guideline used to estimate the gestational age of a fetus based on the size of the fetal rump. It is a simple, yet useful tool for obstetricians and healthcare providers to assess fetal development and identify potential growth restriction or other complications.

**Why the Correct Answer is Right**
Nysten's rule takes into account the distance between the fetal rump and the pubic symphysis, as well as the width of the fetal pelvis. By using these measurements, healthcare providers can estimate the gestational age of the fetus within a few weeks. This is particularly useful in cases where the exact gestational age is uncertain, such as in cases of irregular menstrual cycles or incomplete medical history.

**Why Each Wrong Option is Incorrect**
**Option A:** Nysten's rule is not related to the size of the fetal head.
Nysten's rule focuses on the fetal rump and pelvis, not the head.

**Option B:** Nysten's rule is not a guideline for assessing fetal weight.
While fetal weight is an important consideration in obstetric care, Nysten's rule is specifically used to estimate gestational age, not weight.

**Option C:** Nysten's rule is not a method for diagnosing fetal anomalies.
Nysten's rule is a simple tool for estimating gestational age, not a diagnostic method for fetal anomalies.

**Clinical Pearl / High-Yield Fact**
Nysten's rule is a useful tool for healthcare providers to estimate gestational age, but it should not be used as the sole determinant of fetal development. A thorough ultrasound examination is still necessary to assess fetal growth and development.

**Correct Answer:** D