## **Core Concept**
Rigor mortis, also known as postmortem stiffness, is a well-known phenomenon that occurs after death. In fetuses, the development of rigor mortis is an important consideration in perinatal mortality investigations. The timing of its onset can vary based on several factors including the gestational age of the fetus.

## **Why the Correct Answer is Right**
The correct answer indicates that rigor mortis in a fetus develops after attaining a gestational age of 36 weeks. This is significant because before this gestational age, the muscles in the fetus are not fully developed, which can affect the onset and manifestation of rigor mortis. At around 36 weeks, the muscular system of the fetus is sufficiently mature for rigor mortis to occur similarly to that in newborns and adults, albeit with some differences in presentation and timing.

## **Why Each Wrong Option is Incorrect**
- **Option A:** 28 weeks - This gestational age is too early for the full development of muscles necessary for the typical presentation of rigor mortis. Before 28 weeks, the fetus's muscular system is not mature enough.
- **Option B:** 32 weeks - Although closer, 32 weeks is still considered early for the complete muscular development that would support the typical rigor mortis changes seen in more mature fetuses or newborns.
- **Option D:** 40 weeks - While 40 weeks is a term gestation, rigor mortis can indeed occur before this gestational age in a sufficiently mature fetus.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the development of rigor mortis in fetuses can be an important indicator in the assessment of fetal death and can help in determining the postmortem interval. Understanding the gestational age at which rigor mortis can occur is crucial for forensic and clinical investigations.

## **Correct Answer:** . 36 weeks.