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Molar pregnancy is an abnormal form of pregnancy in which a non-viable fertilized egg implants in the uterus and will fail to come to term. A molar pregnancy is a gestational trophoblastic disease〔(Gestational Trophoblastic Disease ) at American Cancer Society. Last Medical Review: 04/14/2011〕 which grows into a mass in the uterus that has swollen chorionic villi. These villi grow in clusters that resemble grapes.〔(【引用サイトリンク】 hydatidiform mole )〕 A molar pregnancy can develop when fertilized egg had not contained an original maternal nucleus. The products of conception may or may not contain fetal tissue. It is characterized by the presence of a hydatidiform mole (or hydatid mole, mola hydatidosa). Molar pregnancies are categorized as partial moles or complete moles, with the word ''mole'', being used to denote simply a clump of growing tissue, or a ''growth''. A complete mole is caused by a single (incidence is about 90%) or two (incidence is about 10%) sperm combining with an egg which has lost its DNA (the sperm then reduplicates forming a "complete" 46 chromosome set) The genotype is typically 46,XX (diploid) due to subsequent mitosis of the fertilizing sperm, but can also be 46,XY (diploid). 46,YY (diploid) is not observed. In contrast, a partial mole occurs when an egg is fertilized by two sperm or by one sperm which reduplicates itself yielding the genotypes of 69,XXY (triploid) or 92,XXXY (tetraploid).〔 Complete hydatidiform moles have a higher risk of developing into choriocarcinoma — a malignant tumor of trophoblast cells — than do partial moles. The etymology is derived from ''hydatisia'' (Greek "a drop of water"), referring to the watery contents of the cysts, and ''mole'' (from Latin ''mola'' = millstone/false conception).〔Entries HYDATID ''n.'' (''a.'') and MOLE, ''n.''6 in the Oxford English Dictionary online. (http://dictionary.oed.com/ — subscription required.)〕 The term, however, comes from the similar appearance of the cyst to a hydatid cyst in an Echinococcosis.〔http://medical-dictionary.thefreedictionary.com/hydatidiform〕 ==Natural history== An hydatidiform mole is a pregnancy/conceptus in which the placenta contains grapelike vesicles (small sacs) that are usually visible with the naked eye. The vesicles arise by distention of the chorionic villi by fluid. When inspected in the microscope, hyperplasia of the trophoblastic tissue is noted. If left untreated, a hydatidiform mole will almost always end as a spontaneous abortion (miscarriage). Based on morphology, hydatidiform moles can be divided into two types: in ''complete moles'', all the chorionic villi are vesicular, and no sign of embryonic or fetal development is present. In ''partial moles'' some villi are vesicular, whereas others appear more normal, and embryonic/fetal development may be seen but the fetus is always malformed and is never viable. Hydatidiform moles are a common complication of pregnancy, occurring once in every 1000 pregnancies in the US, with much higher rates in Asia (e.g. up to one in 100 pregnancies in Indonesia). In rare cases a hydatidiform mole co-exists in the uterus with a normal, viable fetus. These cases are due to twinning. The uterus contains the products of two conceptions: one with an abnormal placenta and no viable fetus (the mole), and one with a normal placenta and a viable fetus. Under careful surveillance it is often possible for the woman to give birth to the normal child and to be cured of the mole. The etiology of this condition is not completely understood. Potential risk factors may include defects in the egg, abnormalities within the uterus, or nutritional deficiencies. Women under 20 or over 40 years of age have a higher risk. Other risk factors include diets low in protein, folic acid, and carotene. The diploid set of sperm-only DNA means that all chromosomes have sperm-patterned methylation suppression of genes. This leads to overgrowth of the syncytiotrophoblast whereas dual egg-patterned methylation leads to a devotion of resources to the embryo, with an underdeveloped syncytiotrophoblast. This is considered to be the result of evolutionary competition with male genes driving for high investment into the fetus versus female genes driving for resource restriction to maximise the number of children. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「molar pregnancy」の詳細全文を読む スポンサード リンク
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