Fall in the blood pressure during the first two trimesters of pregnancy is normal. This normal fall is most likely due to more than one cause, and important ones are expansion of circulatory system and hormonal effects. Usually, there is approximate fall of around 10 mm of Hg fall in both systolic and diastolic blood pressure by mid-pregnancy, after which the blood pressure slowly starts rising again and reaching near normal levels by the end of third trimester. This normal fall in the blood pressure is protective against preeclampsia.
However, other than this normal physiological low blood pressure, any of the other numerous causes of low blood pressure (dehydration, heart diseases, anemia, etc.) can be concomitantly present during pregnancy. Another important cause of hypotension during pregnancy is the use of antihypertensive drugs for lowering the blood pressure in various hypertensive conditions of the pregnancy (e.g. gestational hypertension, preeclampsia, etc.).
Hypotension or low blood pressure during pregnancy can be without any symptoms or can be associated with symptoms like headaches, dizziness, fainting, etc. The affected woman may also have postural hypotension, characterized by dizziness or fainting when standing up from a sitting or lying down position. The woman may also have supine hypotension, a condition in which the low blood pressure and the symptoms typically occur when lying down on the back. When supine, the gravid uterus puts pressure on the major blood vessels in the abdomen, resulting in fall in blood pressure.
Severe hypotension can have harmful effects on both, the pregnant woman and her developing fetus. Severe hypotension increases the risk of severe nausea and vomiting, anemia and threatened abortion. Moreover, there is an increased risk of preterm labor, low birth weight babies and complications after delivery. However, there is considerable debate on the adverse effects because of presence of many confounding factors. Hypotension has not been found to be associated with congenital malformations in the newborn baby.
Normal asymptomatic low blood pressure during pregnancy requires no treatment. Drugs are avoided if possible and non-pharmacological remedies (like taking plenty of water and salt, mild regular exercises, avoiding standing for long duration, lying down on the side rather than on back, etc.) are tried first. If having symptoms of postural hypotension, then additionally one should be extra careful in standing up from lying down or sitting position (should stand up slowly, and preferable hold on to something while standing up). Compression stockings might also be helpful. Lying on the back (supine) is avoided if supine hypotension is present. Drugs are used only if hypotension is severely symptomatic or poses any risk to mother or the fetus.
Evaluation for any primary cause (heart disease, antihypertensive medications, etc.) is also done. In case of low blood pressure due to some other primary cause, treatment is aimed at the primary cause. For example, if the antihypertensive drugs for the treatment of preeclampsia are causing low blood pressure, their doses might need adjustment or replacement of present drug with another antihypertensive drug could be tried.