# Environmental Pollution and Oxidative Stress: Health Effects During Pregnancy: A Review
## 5 Oxidative Stress and Adverse Obstetric Outcomes  
Several studies have reported the influence of oxidative stress on adverse obstetric outcomes (Table 1). Misz carriage is the unintended loss of a pregnancy that takes place before the normal 20-week gestation period or when the fetus weighs less than 500 g. Studies have shown that the main causative agent of miscarriage (about 50% of cases) is chromosomal abnormalities, while the remaining 50% include uterine abnormalities, infections, diseases, and idiopathic causes [86]. In normal pregnancies, the placenta undergoes an oxidative burst between the 10 th and 12 th weeks, while due to its antioxidant activity, eventually oxidative balance occurs [87]. In contrast, in cases of miscarriage, the placental circulation begins early and sporadically between the eighth and ninth weeks [87; 88]. In many of these cases, low levels of glutathione and total antioxidant capacity [88; 89] as well as signs of apoptosis have been detected, indicating oxidative stress on the trophoblast with consequent loss of pregnancy [90]. Placental antioxidant enzymes are not capable of coping with the increasing production and activity of ROS [89]. Decreased glutathione peroxidase (GPx) activity is associated with a lack of selenium (Se), which can lead to miscarriage [91]. In addition, cases of miscarriage have been linked to decreased activity of various enzymes. A typical example is serum prolidase, the reduced activity of which is negatively connected to an increase in oxidative stress [89]. Another example is serum paraoxanase/arylesterase, whose activity is negatively associated with lipid peroxidation. Thus, reduced enzyme activity in patients with miscarriage is an indication of increased lipid peroxidation [90]. As mentioned earlier, the possibility of a single miscarriage depends on a variety of factors. However, when a second or even a third consecutive miscarriage occurs, the situation is characterized as recurrent. A total of 50% of cases have a known etiology, which includes chromosomal abnormalities of the fetus, but also disorders in the mother that may be anatomical, hormonal, or immune-related. The remaining 50% of cases have still unexplained etiology, with oxidative stress being a fairly likely causative agent [61]. Natural killer (NK) cells have been found in the uterine spiral arteries, which act as regulators of growth and remodeling. Various angiogenic factors are necessary to maintain the proper remodeling of the spiral arteries. In many cases of recurrent miscarriages, increased secretion of angiogenic factors by NK cells in the uterus has been observed [92]. Also, in these cases, an increased number of NK cells was observed in the endometrium, which showed a positive correlation with endometrium vascular density. The increase in NK cells in the uterus increases preimplantation angiogenesis and leads to premature circulation between mother and placenta and consequently an increase in oxidative stress in the early stages of pregnancy [93]. When non-normal placentation occurs, oxidative stress takes place, as well as sores in the syncytiotrophoblast, which bring about the expanded responsiveness of syncytiotrophoblast to oxidative stress during the main trimester and can prompt idiopathic repetitive unsuccessful labors [94]. Aligned with the above-mentioned information, raised degrees of lipid peroxidation and malondialdehydehyde (MDA) have been estimated in patients' blood [95]. Different examinations have connected intermittent premature deliveries with decreased degrees of cancer prevention agent proteins, like GPx, SOD, and catalase, as well as that which could be expected from cell reinforcement enzyme polymorphisms [96]. _Preeclampsia_ is a complex condition that affects the normal blood pressure of pregnant women presenting with hypertension, proteinuria, and edema and is a causal factor of morbidity and mortality of the mother and fetus. It occurs either before (early onset) or after (late onset) the 34 th week of pregnancy.