Anatomic Abnormalities Anatomic abnormalities of both the uterine cervix and the uterine body have been associated with recurrent pregnancy loss (74,75). These anatomic causes may be either congenital or acquired. During development, the uterus forms via the apposition of a portion of bilateral hollow tubes called the m¨ullerian ducts. The dissolution of the walls of these ducts along their site of apposition allows formation of the intrauterine cavity, the intracervical canal, and the upper vagina. Congenital uterine anomalies may, therefore, include incomplete m¨ullerian duct fusion, incomplete septum resorption, and uterine cervical anomalies. Although the causes underlying many of the congenital anomalies of the female reproductive tract are unclear, it has been well documented that prenatal exposure to maternally ingested diethylstilbestrol (DES) results in complex congenital uterine, cervical, and vaginal changes.
Historically, all congenital reproductive tract abnormalities have been linked to both isolated spontaneous pregnancy loss and to recurrent pregnancy loss, although the presence of an intrauterine septum and prenatal exposure to DES demonstrate the strongest associations (76–78). Women with an intrauterine septum may have as high as a 60% risk for spontaneous abortion (79). Uterine septum–related losses most frequently occur during the second trimester (80). However, if an embryo implants into the poorly developed endometrium overlying the uterine septum, abnormal placentation and resultant first trimester losses may occur (81). The most common uterine congenital anomaly associated with in utero DES exposure is hypoplasia, which may contribute to first-or second-trimester spontaneous abortions, incompetent cervix, and premature labor (82,83). Congenital anomalies of the uterine arteries also may contribute to pregnancy loss via adverse alterations in blood ﬂow to the implanted blastocyst and developing placenta (84).
Acquired anatomic anomalies have likewise been linked to both isolated and recurrent pregnancy losses. These abnormalities include such disparate conditions as intrauterine adhesions, uterine fibroids, and endometrial polyps. Endometrium that develops over an intrauterine synechiae or over a fibroid that impinges in the intrauterine cavity (submucous) may be inadequately vascularized (85). This may promote abnormal placentation for any embryo attempting to implant over such lesions. Although data supporting these concepts are limited, this abnormal placentation may lead to spontaneous pregnancy loss. Less clear is the association between intramural fibroids and recurrent pregnancy loss, but it is suggested that large (>5 cm) intramural fibroids are associated with pregnancy loss and that removal improves outcomes (78,86) (see Chapter 15) .