Gestational hypertension or pregnancy-induced hypertension (PIH) is the development of new hypertension in a pregnant woman after 20 weeks gestation without the presence of protein in the urine or other signs of pre-eclampsia. Gestational hypertension is defined as having a blood pressure 
greater than 140/90 on two occasions at least 6 hours apart. Signs and symptoms
No single diagnostic test currently exists to predict the likelihood of developing gestational hypertension. High blood pressure is the major sign in diagnosing gestational hypertension. Some women with gestational hypertension may present asymptomatic, but a number of symptoms are associated with the condition. [1]
Symptoms

Edema
Sudden weight gain
Blurred vision or sensitivity to light
Nausea and vomiting
Persistent headaches
Increased blood pressure

Risk factors
Maternal causes

Obesity
Mothers under 20 or over 40 years old
Past history of diabetes mellitus, hypertension (particularly gestational hypertension) and renal disease
Pre-existing hypertension
Thrombophilias (anti-phospholipid syndrome, protein C/S deficiency, factor V Leiden)
Having donated a kidneyPregnancy

Multiple gestation (twins or triplets, etc.) Placental abnormalities:
Hyperplacentosis: Excessive exposure to chorionic villi
Placental ischemiaFamily history

Family history of pre-eclampsia

Diagnosis
Conditions
There exist several hypertensive states of pregnancy:

Gestational hypertension
Gestational hypertension is usually defined as having a blood pressure higher than 140/90 measured on two separate occasions, more than 6 hours apart, without the presence of protein in the urine and diagnosed after 20 weeks of gestation. Pre-eclampsia
Pre-eclampsia is gestational hypertension plus proteinuria (>300 mg of protein in a 24-hour urine sample). Severe pre-eclampsia involves a blood pressure greater than 160/110, with additional medical signs and symptoms. HELLP syndrome is a type of pre-eclampsia.
For years, it has been the belief of the scientific community that gestational hypertension and pre-eclampsia were relatively unique to humans, although there has been some recent evidence that other primates can also develop similar conditions, albeit due to different underlying mechanisms. The underlying cause of gestational hypertension in humans is commonly believed to be an improperly implanted placenta. Humans have evolved to have a very invasive placenta to facilitate better oxygen transfer from the mother to the fetus, to support the growth of its large brain. Origins of the placenta
The origins of gestational hypertension may lie with the development of humans hemochorial placenta. A hemochorial placenta optimizes the amount of oxygen and nutrients that can be absorbed into the fetal blood supply, while at the same time ensuring rapid diffusion of wastes away from the fetus. This hemochorial placenta differs from lower primates epitheliochorial placentae in the way that it allows the fetal tissues to interact directly with the mothers blood. The hemochorial placenta thereby promotes more rapid diffusion to and from the fetal blood supply.In animals with epitheliochorial placentae such as horses and pigs, the greatest resistance to maternal blood flow in the vascular system was found within the placenta. However, in animals with hemochorial placental structures such as rodents and primates, the vascular resistance in the placenta was low, leading scientists to the conclusion that the greatest resistance to maternal blood flow is found elsewhere in the maternal vascular system.