As raised blood- pressure (hypertension) is a vast subject, we shall try to explain it in as concise a way as possible.
How many types of hypertension (in pregnancy) are there?
There are two broad groups. These are:
Pre-existing hypertension can also be termed "chronic hypertension" or, when the cause is unknown (as is mostly the case), "essential hypertension". If you happen to suffer from hypertension, you will encounter these terms many times.
Pregnancy-induced hypertension is more popularly known as pre-eclampsia and was also once known as "toxaemia of pregnancy", "pre-eclamptic toxaemia" (PET) and, across the Atlantic, "EPH gestosis". These terms may crop up occasionally but they are largely historical now.
Hypertension and pre-eclampsia
What causes pre-existing (chronic) hypertension?
Chronic hypertension is uncommon in the young and starts to become a significant feature in the late thirties and early forties. For the majority of people known to have chronic hypertension, the cause is unknown. That is why this is known as essential hypertension. For the remainder, the hypertension may be associated with such diseases as diabetes, kidney disease or SLE. Obesity is an associated factor.
What causes pre-eclampsia?
As mentioned before, pre-eclampsia is a purely pregnancy related condition. It does not occur otherwise.
We, do, however, have to be careful here. The classification of hypertensive conditions in pregnancy is not mutually exclusive. Women with pre-existing hypertension are certainly more at risk of developing pre-eclampsia. That is, their uncomplicated hypertension may evolve into pre-eclampsia. Otherwise, the cause of pre-eclampsia remains unknown. The majority of pregnant women who develop pre-eclamsia will have no pre-existing hypertension.
What is the difference between hypertension and pre-eclampsia?
One is part of the other.
Hypertension is simply above normal blood pressure. In pregnancy; this is generally taken to be a blood pressure of 140/90 or above. This is a convenient cut-off and not always significant. Its importance differs among individuals.
Pre-eclampsia is a syndrome which includes hypertension and loss of protein in the urine. Almost invariably (but not necessarily) there is oedema.
Is chronic hypertension a risk factor in pregnancy?
Yes, though not to the same extent as pre-eclampsia.
Firstly, there is a tendency among hypertensive expectant mothers to have abnormal blood vessel development in the uterus and placenta. This may mean that the fetus does not get sufficient nutrients and oxygen and the intrauterine growth may be restricted to some degree. This, however, is uncommon in the presence of well-controlled hypertension and in the absence of pre-eclampsia.
Secondly, chronic hypertension could evolve into pre-eclampsia, where the normal antihypertensive medication is ineffective and which may lead to premature delivery.
Thirdly, hypertension is associated with an increased risk of placental abruption, which is the premature separation of the afterbirth, with potentially grave consequences to the fetus and even the mother.