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正常情况下,精子和卵子应该在输卵管中结合成为受精卵。受精卵又必须种植到子宫内膜中并形成胎盘后才能发育为胎儿。一般认为,种植于子宫内膜前的胚胎细胞对外界环境有相对的抵抗力,各种外因不致引起畸形,这段时间是指受精后的13～14天之内。从受孕的第13～14天起,胚胎的绒毛中开始有血管形成,子宫内膜的螺旋动脉伸入绒毛间隙中,到第4～5周才开始建立胎盘循环并逐渐完善。然而,母体血循环与胎儿的血循环始终并不直接相通。胎血在绒毛内循环,孕妇的血液在绒毛外的绒毛间隙中循环,两者之间隔着绒毛壁的两层细胞、少许间质和绒毛中的血管壁。大多数药物经过母体进入胎儿的途径是: 药物吸收进入母血后,可经过子宫动
Under normal circumstances, sperm and eggs should be combined in the fallopian tubes into fertilized eggs. Fertilized eggs have to be planted into the endometrium and the formation of the placenta to develop into a fetus. Generally believed that the embryo cells planted in the endometrium before the external environment has a relative resistance, a variety of external causes do not cause deformity, this time refers to the fertilization after 13 to 14 days. From the 13th to 14th day of pregnancy, blood vessels formed in the villus of the embryo, and the spiral artery of the endometrium extended into the villus space, and the placental circulation began to be established and gradually improved after the 4th to 5th weeks. However, maternal blood circulation and the fetus’s blood circulation is always not directly connected. Fetal blood circulates in the villi, and the pregnant woman’s blood circulates in the villous gaps outside the villi, separating the two layers of the villous wall cells, the interstitium, and the vasculature in the villi. Most drugs through the mother into the fetus is: drug absorption into the maternal blood, can move through the uterus