Polycystic ovary syndrome (PCOS) is a common reproductive endocrine disorder with a prevalence of 4%-12% in women of reproductive age. Its etiology is complex, with pathophysiological changes involving reproduction, metabolism and even chronic inflammation, and it has various clinical manifestations such as infertility, obesity, hyperinsulinism, hyperandrogenism, hypofibrinolysis and hypertriglyceridemia, and is prone to obstetric complications. To review the progress of research on the causes and prevention of pregnancy loss in patients with PCOS. Pregnancy loss rates in patients with PCOS Women with PCOS are vulnerable to obstetric complications, including spontaneous abortion, gestational diabetes mellitus (GDM), gestational hypertensive disorders, and preterm delivery. Studies have reported an early spontaneous abortion rate of 20% to 50% in patients with PCOS. Patients with PCOS who underwent in vitro fertilization-embryo transfer (IVF-ET) had an early miscarriage rate of 20%-35%, which was higher than that of same-age controls. Most studies have concluded that patients with PCOS have an increased rate of spontaneous abortion after pregnancy, but there is also disagreement. Haakova et al. found no increase in spontaneous abortion rates in the PCOS group after matching for age and body mass, and Meta-analysis also showed that abortion rates were not higher in the PCOS group than in a randomized control group selected from a comparable population. Therefore, it is not certain that the rate of pregnancy loss is higher in women with PCOS than in normal controls. the causes of pregnancy loss in patients with PCOS are unclear. in addition to common miscarriage factors such as chromosomal abnormalities, anatomical abnormalities, infection, and immunity, endocrine metabolic abnormalities such as abnormal embryonic quality, obesity, insulin resistance, hyperluteinizing hormone (LH)emia, hyperandrogenemia, hyperhomocysteinemia and fibrinolytic activity All of these may be associated with the high rate of miscarriage in PCOS. The following discusses the appropriate preventive measures in relation to the causes of pregnancy loss in patients with PCOS. The causes of early pregnancy loss in patients with PCOS are discussed below. i. PCOS may affect the quality of oocytes and embryos. This may affect oocyte development as well as embryo quality, which may be associated with their low embryo implantation rate and high miscarriage rate. The abnormal microenvironment and its adverse effects on pregnancy include: abnormal negative feedback of serum estradiol (E2) and progesterone, which leads to reduced frequency of pituitary LH peaks and causes impaired follicular development and maturation; hyperandrogenism and hyperinsulin may cause miscarriage by directly damaging the egg and early embryo; and disruption of growth factors, which affects the synthesis of follicular steroid hormones and affects normal follicular development, resulting in decreased embryo quality leading to miscarriage. Second, patients may have decreased endometrial tolerance The key link affecting miscarriage in patients with PCOS may be the condition of the endometrium, and various endocrine hormones in PCOS patients may cause the endometrium to show hyperplasia by binding to endometrial receptors. Genome-wide testing revealed that most gene expression in the endometrium of PCOS patients during the implantation window was downregulated, presumably resulting in decreased endometrial tolerance. decreased endometrial tolerance in PCOS patients may be associated with the following five factors. 1. Progesterone/E2 dysregulation Progesterone/E2 dysregulation causes abnormal endometrial regulation mechanism in PCOS patients and the lack of cyclic changes in estrogen and progesterone receptors, resulting in the endometrial development and embryonic development out of sync, causing embryonic implantation disorders and early spontaneous abortion. 2. High androgens High androgens are one of the main pathological features of PCOS. Studies have shown that high levels of androgens before pregnancy in PCOS patients are closely related to miscarriage, and that high expression of androgen receptors in the endometrial glandular epithelium during the secretory phase of PCOS reduces the expression of integrins and the molecular marker of endometrial tolerance, homologous frame gene A10 (HOXA10). This affects the growth of the endometrium and thus affects embryo implantation and causes miscarriage. Many studies have found that increased LH levels are an important factor in early miscarriage in patients with PCOS. The literature reports an increased rate of miscarriage in women with high basal LH levels. However, the mechanism that causes miscarriage is not well understood. The possible mechanism is that increased LH causes a hyperandrogenic environment in the follicle and premature follicular maturation and early completion of the second mitosis, which affects oocyte and embryo quality, fertilization and the process of implantation, leading to early miscarriage. 4. insulin resistance and hyperinsulinemia Insulin resistance and hyperinsulinemia are one of the basic features of abnormal glucose metabolism in PCOS patients and may be the central link in the occurrence of miscarriage in PCOS patients. insulin resistance also exists locally in the endometrium of PCOS patients, causing abnormal endometrial proliferation and functional defects in PCOS. High insulin reduces the level of insulin-like growth factor binding protein 1 (IGFBP- 1), which impairs the endometrial epithelial and mesenchymal function in PCOS patients in early pregnancy and affects embryo implantation and leads to miscarriage. The incidence of PCOS in obese women is 35%~60%, much higher than that in women with normal body mass, and obese patients with PCOS (body mass index >25 kg/m2) account for about 40%~60%, and obesity aggravates the degree of insulin resistance and hyperinsulinemia in patients. 5. Increased hyperhomocysteinemia Homocysteine may locally interfere with blood flow and vascular integrity at the uterine maternal-fetal interface by increasing oxidative stress in the vascular endothelium, making the endometrial environment unfavorable for embryonic implantation or increasing the likelihood of early spontaneous abortion. The presence of high PAI activity and decreased fibrinolytic activity in women with PCOS may lead to microthrombosis at the maternal-fetal interface in early pregnancy, resulting in inadequate fetal blood supply and miscarriage due to obstruction of trophoblast growth. The incidence of miscarriage in PCOS patients treated with assisted reproductive technology (ART) for pregnancy is significantly increased, which may be related to multiple factors such as elevated hormone levels, decreased egg quality, endometrial damage and embryonic chromosomal abnormalities due to superovulation and the assisted conception process itself. Moreover, the use of ovulation-promoting drugs may lead to multiple pregnancies and pregnancy loss. In addition, the occurrence of ovarian hyperstimulation syndrome (OHSS) is a detrimental factor for the maintenance of pregnancy. In addition, the psychological status of PCOS patients may affect pregnancy outcome. It has been found that patients with PCOS may experience a lack of confidence or depression, which may also contribute to the occurrence of early miscarriage. Causes of late pregnancy loss and preventive measures PCOS patients are prone to complications during pregnancy such as GDM, gestational hypertensive disorders, multiple pregnancies and the birth of low-body mass children. Among them, preeclampsia and GDM are the most common complications of PCOS pregnancy and may cause late pregnancy loss in PCOS patients, but the mechanism is not clear yet. Measures to prevent pregnancy loss in patients with PCOS are as follows. 1. Improve lifestyle. The primary preventive measures for pregnancy loss in PCOS patients are mainly lifestyle adjustment and improvement, establishing a regular rhythm of life, reasonable dietary structure, restricting caloric intake, actively controlling body mass, strengthening physical activity and exercise and adjusting psychological status, all of which have the effect of increasing the pregnancy rate. Some studies have suggested that fitness treatments can improve pregnancy and reduce the abortion rate in PCOS patients. For obese PCOS patients, it is recommended that they should reduce the miscarriage rate by losing weight before preparing for pregnancy. 2. Insulin sensitizers. Improving insulin sensitivity treatment is a key measure to prevent pregnancy loss in PCOS patients. insulin and PAI- 1 were significantly decreased in PCOS patients after taking insulin sensitizers, and the ovulation rate and miscarriage rate in PCOS patients were significantly improved. The most commonly used insulin sensitizer is metformin, which can reduce insulin and testosterone levels by improving insulin sensitivity in the liver, improve the local endocrine environment of the ovary, and improve egg quality; it can reduce the activity and level of plasma homocysteine and PAI- 1, and also increase the level of serum IGFBP- 1, increase the vascular blood supply to the uterus, improve the tolerance of the endometrium, and facilitate Metformin also promotes the formation of uterine arteries, decreases plasma endothelin 1 levels, increases serum placental protein 14 concentration during the luteal phase, decreases androgen and LH levels, and in some patients also decreases body mass. The study suggested that metformin treatment before pregnancy and early pregnancy did not increase the rate of fetal malformation, and the application of metformin continued throughout pregnancy significantly reduced the rate of miscarriage, and no adverse effects of metformin on the fetus were found. 3. Pre-treatment with oral contraceptives before ovulation promotion. The commonly used oral contraceptive pill is Daing-35, which can anti-androgen, inhibit the secretion of LH, improve the intrauterine environment and systemic endocrine status, thus reducing the incidence of miscarriage. Adjustment of the endometrium. In PCOS patients with estrogen and progesterone receptor deficiency, progesterone supplementation can only increase blood progesterone levels, but not the uptake of progesterone by endometrial cells. The progesterone receptor level in the endometrial tissue should be increased first to ensure normal endometrial metaphase and avoid miscarriage. It was found that oral administration of small doses of aspirin increased the number of endometrial glands, glandular area and interstitial area as well as the glandular/interstitial ratio, and increased the amount of estrogen and progesterone receptors. In patients with PCOS who have a history of spontaneous abortion, it is recommended that endometrial estrogen and progesterone receptor levels be measured first and that the estrogen and progesterone receptor levels in the endometrial tissue be increased while correcting the abnormal endocrine status in order to improve pregnancy outcomes. Prevention strategies for miscarriage after ART in patients with PCOS. For PCOS patients receiving ART, pretreatment with oral contraceptives and insulin sensitizers prior to ovulation promotion, selection of an appropriate ovarian stimulation protocol, timing of egg retrieval and luteal support are helpful in reducing the miscarriage rate. antoine et al. found that the miscarriage rate in PCOS tended to decrease after the use of gonadotropin-releasing hormone antagonist (GnRHA) to suppress LH secretion from the pituitary. Aggressive prevention of pregnancy complications. Oral insulin sensitizers can reduce the risk of pregnancy complications. The literature reports that metformin administration throughout pregnancy in patients with PCOS not only reduces body mass, blood insulin levels, insulin resistance, insulin release, and blood androgen rates. The literature reports that treatment with metformin during pregnancy (2,550 mg/d) reduces the likelihood of GDM by 90% in non-diabetic individuals with PCOS. There is very limited information on the correlation between GDM and gestational hypertension, and it is recommended that patients with PCOS, especially those who are obese, be screened for related complications, such as close monitoring of minimal blood pressure and blood glucose, controlling pregnancy body mass, and adjusting diet, etc. Patients with high-risk factors, especially those with a family history of diabetes, should be carefully evaluated for their risk of pregnancy complications. maternal GDM is prone to complications Gestational hypertension, amniotic fluid abnormalities, etc. The most serious complication is ketoacidosis, so GDM should be correctly diagnosed and controlled early, and attention should be paid to follow-up and treatment to reduce the level of adverse pregnancy outcomes, and also reduce the occurrence of GDM, preeclampsia, and giant fetus.