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Choice A reason: Poor maternal weight gain is a risk factor for IUGR, as it indicates inadequate nutrition and fetal growth.

Choice B reason: Premature rupture of membranes is not a risk factor for IUGR, as it does not affect the placental function or blood flow. It is a complication of pregnancy that can lead to infection, preterm labor, or cord prolapse.

Choice C reason: Smoking is a risk factor for IUGR, as it reduces the oxygen and nutrient delivery to the fetus and causes vasoconstriction of the placental vessels.

Choice D reason: Gestational hypertension is a risk factor for IUGR, as it impairs the placental perfusion and causes fetal hypoxia and acidosis.

Choice A reason: This is not an appropriate response by the nurse, as an ultrasound is not a screening tool for spina bifida. Spina bifida is a birth defect that occurs when the spine and spinal cord do not form properly. It can be detected by a blood test that measures the level of alpha-fetoprotein (AFP) or by a detailed ultrasound that shows the spine and the brain.

Choice B reason: This is not an appropriate response by the nurse, as an ultrasound is not necessary to determine if there is more than one fetus. Multiple fetuses can be detected by other methods, such as listening to the fetal heartbeats, measuring the fundal height, or feeling the fetal movements.

Choice C reason: This is an appropriate response by the nurse, as an ultrasound assists in identifying the location of the placenta and fetus. This is important for an amniocentesis, which is a procedure that involves inserting a needle through the abdomen and the uterus to collect a sample of amniotic fluid. The ultrasound helps to guide the needle and avoid injuring the placenta or the fetus.

Choice D reason: This is not an appropriate response by the nurse, as an ultrasound is not useful for estimating fetal age at 36 weeks of gestation. Fetal age can be estimated by an ultrasound in the first trimester, when the fetus is growing at a predictable rate and has distinct features. However, in the third trimester, the fetus grows at different rates and has more variations in size and shape. Therefore, an ultrasound is less accurate and reliable for estimating fetal age at this stage.