Which of the following can cause decreased fetal variability?

Decreased fetal variability can be attributed to several factors, including the effects of certain substances on the mother's body and, consequently, the fetus. In this context, opioids, magnesium, and tobacco have characteristics that can lead to reduced fetal heart rate variability.

Opioids, used for pain relief, can depress the central nervous system, which may result in decreased activity and variability in fetal heart rate. This is particularly relevant as fetal variability is a sign of well-being and reflects adequate neurologic function and responsiveness in the fetus.

Magnesium, often used in obstetric care to manage conditions like preeclampsia, can also have a calming effect on the fetal nervous system, potentially leading to diminished heart rate variability. It acts as a central nervous system depressant that can reduce muscular activity.

Tobacco is known to have adverse effects on pregnancy, including reducing oxygen supply to the fetus. This can lead to a compromised fetal condition and result in reduced variability in heart rate readings due to stress on the fetus.

In contrast, other choices include factors that do not consistently lead to decreased variability or can even promote normal fetal activity and response. For instance, hydration and rest can generally improve maternal and fetal well-being, and stress and anxiety might actually increase