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Implantation of the blastocyst begins at the end of the first week and is completed by the end of the second week. The cellular and molecular events relating to implantation are complex. It involves a receptive endometrium and hormonal factors, such as estrogen, progesterone, prolactin, as well as cell adhesion molecules, growth factors, and HOX genes, Implantation may be summarized as follows:

* The zona pellucida degenerates (day 5). Its disappearance results from enlargement of the blastocyst and degeneration caused by enzymatic lysis. The lytic enzymes are released from the acrosomes of the sperms that surround and partially penetrate the zona pellucida.
* The blastocyst adheres to the endometrial epithelium (day 6).
* The trophoblast differentiates into two layers: the syncytiotrophoblast and cytotrophoblast (day 7).
* The syncytiotrophoblast erodes endometrial tissues, and the blastocyst starts to embed in the endometrium (day 8).
* Blood-filled lacunae appear in the syncytiotrophoblast (day 9).
* The blastocyst sinks beneath the endometrial epithelium, and the defect is filled by a closing plug (day 10).
* Lacunar networks form by fusion of adjacent lacunae (days 10 and 11).
* The syncytiotrophoblast erodes endometrial blood vessels, allowing maternal blood to seep in and out of lacunar networks, thereby establishing a uteroplacental circulation (days 11 and 12).
* The defect in the endometrial epithelium is repaired (days 12 and 13).
* Primary chorionic villi develop (days 13 and 14).
Placenta Previa
Body_ID: B003002
Implantation of a blastocyst in the inferior segment of the uterus near the internal os results in placenta previa, a placenta that partially or completely covers the os Placenta previa may cause bleeding because of premature separation of the placenta during pregnancy or at delivery of the fetus.

Spontaneous Abortion of Embryos
Most spontaneous abortions of embryos occur during the first 3 weeks. Sporadic and recurrent spontaneous abortions are two of the most common gynecologic problems. The frequency of early abortions is difficult to establish because they often occur before a woman is aware that she is pregnant. An abortion occurring several days after the first missed period is very likely to be mistaken for a delayed menstruation. Detection of a conceptus in the menses (menstrual blood) is very difficult because of its small size.
Body_ID: PB003008
Study of most early spontaneous abortions resulting from medical problems reveals abnormal conceptuses. More than 50% of all known spontaneous abortions result from chromosomal abnormalities. The higher incidence of early abortions in older women probably results from the increasing frequency of nondisjunction during oogenesis (see Chapter 2). It has been estimated that from 30% to 50% of all zygotes never develop into blastocysts and implant. Failure of blastocysts to implant may result from a poorly developed endometrium; however, in many cases, there are probably lethal chromosomal abnormalities in the embryo. There is a higher incidence of spontaneous abortion of fetuses with neural tube defects, cleft lip, and cleft palate.

Inhibition of Implantation
Body_ID: B003004
The administration of relatively large doses of progestins and/or estrogens ("morning-after pills") for several days, beginning shortly after unprotected sexual intercourse, usually does not prevent fertilization but often prevents implantation of the blastocyst. A high dose of diethylstilbestrol, given daily for 5 to 6 days, may also accelerate passage of the cleaving zygote along the uterine tube. Normally, the endometrium progresses to the secretory phase of the menstrual cycle as the zygote forms, undergoes cleavage, and enters the uterus. The large amount of estrogen disturbs the normal balance between estrogen and progesterone that is necessary for preparation of the endometrium for implantation.
Body_ID: PB003010
An intrauterine device inserted into the uterus through the vagina and cervix usually interferes with implantation by causing a local inflammatory reaction. Some intrauterine devices contain progesterone that is slowly released and interferes with the development of the endometrium so that implantation does not usually occur.

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