The most common cause of decreased variability in the fetal heart rate (FHR) that lasts 30 minutes or less is:

Choice A reason: A previous birth of a large infant (macrosomia) is a risk factor for gestational diabetes mellitus (GDM). A large infant may indicate that the mother had high blood glucose levels during pregnancy, which can cause the fetus to grow larger than normal. Women who have had a large infant are more likely to develop GDM in subsequent pregnancies.

Choice B reason: Underweight before pregnancy is not a risk factor for GDM. In fact, being overweight or obese before pregnancy is a risk factor for GDM, as it increases insulin resistance and makes it harder for the body to use glucose effectively.

Choice C reason: A previous diagnosis of type 2 diabetes mellitus is not a risk factor for GDM. It is a contraindication for GDM, as it means that the woman already has diabetes before pregnancy. GDM is a condition that develops during pregnancy and usually resolves after delivery.

Choice D reason: Maternal age younger than 25 years is not a risk factor for GDM. In fact, being older than 25 years is a risk factor for GDM, as it increases the risk of insulin resistance and other metabolic changes that can affect glucose tolerance.

Choice A reason: This is not a good intervention, as it may worsen the symptoms of faintness, dizziness, and agitation. Standing up can cause a sudden drop in blood pressure (orthostatic hypotension), which can reduce the blood flow to the brain and the fetus. This can cause lightheadedness, blurred vision, and loss of consciousness in the woman, as well as fetal distress or hypoxia.

Choice B reason: This is not a good intervention, as it may also worsen the symptoms of faintness, dizziness, and agitation. Lying supine can cause compression of the inferior vena cava (a large vein that returns blood to the heart) by the gravid uterus, which can reduce the cardiac output (the amount of blood pumped by the heart) and the blood pressure. This can cause nausea, sweating, and visual disturbances in the woman, as well as fetal distress or hypoxia.

Choice C reason: This is not a good intervention, as it may not improve the symptoms of faintness, dizziness, and agitation. Sitting down and holding the arm in a dependent position can lower the blood pressure in the arm, but not in the rest of the body. This can cause inaccurate readings of the blood pressure and delay the detection of hypotension or hypertension. This can also cause discomfort and pain in the arm due to impaired circulation.

Choice D reason: This is the best intervention, as it can relieve the symptoms of faintness, dizziness, and agitation by improving the blood flow to the brain and the fetus. Turning to the left side can reduce the pressure of the uterus on the inferior vena cava and increase the cardiac output and the blood pressure. This can also optimize the placental perfusion (the blood flow to the placenta) and the fetal oxygenation.