Which of the following statements about pyelonephritis in pregnancy is correct?

The correct answer is B: Untreated pyelonephritis in pregnancy can increase the risk of preterm labor and low birth weight. Pyelonephritis is a serious infection of the kidneys that can lead to complications during pregnancy. If left untreated, it can result in adverse outcomes such as preterm labor and low birth weight. Treatment is essential to prevent these risks and ensure the health of both the mother and the fetus.
Choice A is incorrect because pyelonephritis can indeed pose risks to the fetus. Choice C is incorrect as pyelonephritis is not a common condition during pregnancy. Choice D is incorrect as pyelonephritis is a significant infection that requires timely treatment.