This chapter is on physiology of pregnancy. Pregnancy is characterized by progesterone-mediated hyperemia and edema of mucosal surfaces. Pregnant women tend to have more nasal congestion. Pregnant patients in intensive care unit (ICU) differ from other patients, due to complex physiological changes of pregnancy and unique obstetrics problems. Respiratory changes, cardiac changes and renal changes are the physiological changes during pregnancy. The care of pregnant patients in the ICU requires a formal understanding of the uteroplacental unit. Placenta is an inefficient gas exchange organ compared to that of lung and it is crucial for the critical care specialist to understand and know the mechanisms of gas exchange across placenta to maintain adequate fetal oxygenation. Respiratory and gas exchange, nutrition for the fetus and waste elimination are three main functions of the placenta. Pharmacotherapy during pregnancy requires consideration of the pharmacologic and teratogenic effects of drugs on the embryo or fetus. The major risk period for teratogenesis is the first 10 weeks of gestation.