怀孕从受精开始到胎儿出生结束，是一个漫长的发育过程。妊娠失败，或称为妊娠丢失或流产，可以发生在妊娠的不同阶段，并且可能涉及许多不同的病理生理学机制。孕期是一个发展的过程，很难去定义每个不同阶段的妊娠情况。不过，早期妊娠通常是指着床后至第三个月的妊娠阶段，这可以进一步分为:(I):临床前妊娠，只能通过HCG水平涨值来辨别，以及(II)超声或组织学证据可以证明的临床妊娠。

　　In the past, focus was given primarily to those pregcy failures occurring after a clinically recognized gestation, as preclinical pregcy losses would commonly go unnoticed or present merely as a late period. However, with the development of over-the-counter pregcy testing kits for early pregcy recognition, and the widespread use of ART, the focus of interest in pregcy failure has gradually shifted to the earlier stages of gestation. As a result, it has become increasingly evident that even women with normal fertility appear to have a large number of pregcy losses occurring at a very early gestational stage.

　　在过去，临床妊娠后的流产现象备受关注，原因是临床前妊娠失败容易容易受到忽视或仅在较晚的时期出现。然而，随着用于早期妊娠鉴别的非处方妊娠检测试剂盒的发展，以及辅助生殖技术的广泛使用，对妊娠失败的关注焦点已逐渐转移到妊娠的早期阶段。结果显示，即使是生育能力正常的妇女，也会在妊娠早期频发流产。

　　Roberts and Lowe were among the first to demonstrate the scale of early pregcy wastage, by using the registered number of births in England and estimating the number of fertile ovulatory cycles in married couples having unprotected intercourse. They calculated that only 22% of cycles exposed to the risk of pregcy actually resulted in a live birth (1). Because the sporadic miscarriage rate of pregcies is far less than 78%, this finding implied that the vast majority of pregcies were being lost at a very early stage, before they were clinically recognized. In fact, a review of the literature suggests that of all pregcies, approximately 30% fail prior to implantation, a further 30% fail following implantation but prior to clinical evidence of pregcy, and about 10% suffer clinical miscarriages. This means that only 30% of all conceptions in fertile women will result in a live birth (2).

　　Roberts和Lowe是两位最早通过使用英国登记的出生人数，并估算已婚夫妇无保护措施性行为的可育排卵周期数，来说明早期妊娠流产规模的研究人员。结果表明只有22%的周期最终妊娠并且有婴儿出生(数据参考见1)。由于妊娠的偶发性流产率远低于78%，这一发现意味着，绝大多数的妊娠是在临床确认之前就流产了。事实上，文献回顾表明，在所有妊娠中，大约30%的胚胎没有着床，另有30%在着床后和临床妊娠确认前丢失，约10%遭遇临床流产。这意味着，在育龄女性中，女性的所有受孕中只有30%的概率会诞生一名婴儿(数据参考见2)。

　　References 数据参考：

　　1. Roberts CJ, Lowe CR. Where have all conceptions gone ? Lancet 1975; 1:498-499

　　2. Macklen NS, Geraedts JP, Fauser BC. Conception to ongoing pregcy : the “black box” of early pregcy loss. Hum Reprod Update 2002; 8:333-343.