Placental defence is considered sufficient to control lipid peroxidation in pregnancy.

Oxidative stress is characterized by an overweight of pro-oxidants against antioxidant forces and is associated with atherosclerosis, aging, and reproductive complications. Placenta is the major source of pro-oxidant agents, antioxidant enzyme-systems, and hormones and is able to keep the lipid peroxidation under control in normal pregnancy. As oxidation processes are increased in pregnancy, we would expect a shortening of lag phase of low density lipoprotein (LDL)-oxidation. LDL oxidation assessed by measuring conjugated diene formation is a marker for the early part of lipid peroxidation process and the balance of pro-oxidants and antioxidants influences length of lag phase. Our results show that resistance of LDL oxidation is unchanged during normal pregnancy in the second and third trimester. Only antioxidants are able to protect LDL. Estriol is an antioxidant, increases the lag-phase of LDL-oxidation in vitro, and its serum concentration raises enormous during late pregnancy. Thus the biological role of high levels of estriol during pregnancy may be part of the self-protection to limit oxidative damages.