Ironically, hypertension in the mother so blocks the normal nutritional exchange that the fetus has the opposite problem of hypotension (low blood pressure). Untreated, the same damaging effects to the blood vessels in the expectant mother can also damage the blood supply involved with placental exchange of oxygen and nutrition from mother to baby. This can age the placenta prematurely.
The well-known result in hypertensive pregnancies is intrauterine growth restriction or reduction (IUGR) which results in small babies and oligohydramnios, which can endanger the fetal kidneys by decreasing the amount of urine the unborn baby produces—the urine being the most significant portion of amniotic fluid. ACE inhibitors will exaggerate this danger considerably, which is why it is recommended that ACE inhibitors be discontinued the moment pregnancy is diagnosed.
One relief to the newly pregnant patients who have been on ACE inhibitors is that the danger seems to be in the later point of pregnancy, so getting off of them in early pregnancy is probably all that need be done to relieve any worries. It is then quite safe to continue with any of the older treatments. As the ACOG (American College of Obstetricians and Gynecologists) states, "With the exception of the ACE inhibitors, there is no need to discontinue any of the other drugs commonly used to treat hypertension in a pregnant patient whose blood pressure is well controlled."
A particular complication of pregnancy is called pregnancy-induced hypertension (PIH), which is quite different from the "chronic" hypertension discussed above. PIH is the new term for the old-termed "preeclampsia" and the even older term "toxemia of pregnancy."
Although the exact cause of PIH is unknown, it seems to be an immunologic rejection of the pregnancy, causing the body to have a hostile tissue-graft reaction to the baby. This condition is much more dangerous than chronic hypertension, because there is considerably more alteration in the maternal body than just high blood pressure.
There's a whole chemical shift of maladaptative reactions that can—in very severe and rare cases—even lead to seizures and death in the pregnant patient. Chronic hypertension, on the other hand—even in pregnancy—is a slow-growing problem that allows plenty of time for management that seldom interferes with bringing a pregnancy to term.
If chronic hypertension is the tortoise, PIH is the hare. There are no real winners in this race, but as an obstetrician I'd much rather team with the tortoise.