Cervico-isthmic Pregnancy

Implantation of the gestational sac between the histological internal os (transition point from the endocervical mucosa to the isthmic mucosa) and the anatomical internal os (transition point from the isthmus to the corpus of the uterus, located 5 to 16 mm cranially to the histological os)¹. It is different from the cervical pregnancy where location is completely within the endocervical mucosa.

O43.9 Placental disorder, unspecified.

Incidence is unknown, although cervico-isthmic pregnancy is supposed to be the least frequent among low-lying ectopic pregnancies². Most cases detected in the first trimester are misdiagnosed and treated as cervical pregnancies. In the last 40 years, 19 cases of cervico-isthmic pregnancies developing beyond 20 weeks were described in the literature^1,3.

Among several causative hypothesis, a key role is played by the internal cervical os which is weakened (i.e. due to cervical surgical procedures). Implantation in the thin isthmic mucosa and possibly in the cervix, which lacks a protective decidua basalis, may lead to abnormal trophoblastic invasion and subsequently to the development of placenta accreta spectrum disorders. From the 12th week onward, the gestational sac expands cranially to the uterine fundus, but not completely obliterating it, and distally to the cervix, with progressive effacement and dilation. The absence of contractility of the cervix and isthmus may cause massive bleeding antenatally and upon termination or delivery resulting in salvage hysterectomy^1,4-5.

There is no identified cause for cervico-isthmic pregnancies. Associated conditions include uterine curettage, history of miscarriages, previous cesarean deliveries, uterine fibroids and assisted reproductive techniques^1,3-5.

Cervico-isthmic pregnancies are almost universally associated with placenta accreta spectrum disorders^1,3.

The diagnosis is made when there is an image of a gestational sac in the isthmus, the cervix is closed and preserved and more than half of the uterine cavity is not involved by the gestational sac implantation¹.

Differential diagnosis in the first trimester includes miscarriage and cervical pregnancy. If the gestational sac moves upon gentle pressure with the ultrasound probe (sliding sign), it is probably a miscarriage in progress. In the case of a viable pregnancy, the presence of a well-formed and closed cervical canal is clinically and sonographically more in keeping with a cervico-isthmic rather than a cervical pregnancy. Although challenging, this distinction is of the utmost importance, as a cervical pregnancy rarely progresses past 20 weeks of gestational age and may be complicated by catastrophic hemorrhage and hysterectomy, especially if not terminated at an early stage. On the contrary, cervico-isthmic pregnancies are frequently viable and could lead to consistent perinatal morbidity⁷. Magnetic resonance imaging may aid in this differential diagnosis^6,7.

This entity constitutes a very high-risk pregnancy, as it usually results in painless and profuse bleeding, progressive cervical effacement and dilation, preterm premature rupture of membranes or preterm labour. Placenta accreta spectrum disorder, with potentially massive hemorrhage upon delivery, often requiring hysterectomy and tranfusion of blood products, is almost always present^{1, 3-4}.

If the diagnosis is made in the first trimester, termination of pregnancy should be considered, especially if there is a desire for fertility preservation. In this case, management is similar to cervical pregnancies. Treatments described include methotrexate (systemic or intra-amniotic), dilation and curettage with balloon tamponade, hysteroscopic resection and uterine artery ligation or embolization^4,7. There is not a standardized management in the situations in which the woman wishes to continue the pregnancy, owing to the rarity of this condition. It is advisable to screen for placenta accreta spectrum disorders and to prepare for massive and potentially life-threatening hemorrhage at delivery. Out of the 19 cases described in the literature, only 4 allowed for uterine sparing treatment^1,4,8-11.

References:

1. ^{Strobelt N, Locatelli A, Ratti M, Ghidini A. Cervico-isthmic pregnancy: a case report, critical reappraisal of the diagnostic criteria, and reassessment of the outcome. Acta Obstet Gynecol Scand. 2001;80(6):586-8.}
2. ^{Tsai SW, Huang KH, Ou YC, Hsu TY, Wang CB, Chang MS, et al. Low-lying-implantation ectopic pregnancy: a cluster of cesarean scar, cervico-isthmus, and cervical ectopic pregnancies in the first trimester. Taiwan J Obstet Gynecol. 2013;52(4):505-11.}
3. ^{Honda T, Hasegawa M, Nakahori T, Maeda A, Sai R, Takata H, et al. Perinatal management of cervicoisthmic pregnancy. J Obstet Gynaecol Res. 2005;31(4):332-6.}
4. ^{Avery DM, Wells MA, Harper DM. Cervico-isthmic corporeal pregnancy with delivery at term: a review of the literature with a case report. Obstet Gynecol Surv. 2009;64(5):335-44.}
5. ^{Loureiro T, Cunha M, Marques E, Araújo ML, Montenegro N, Laurini R, et al. Non-viable cervico-isthmic pregnancy: the importance of an accurate sonographic diagnosis to preserve fertility. Fetal Diagn Ther. 2003;18(5):289-91.}
6. ^{Kumar N, Agrawal S, Das V, Agrawal A. Cervical Pregnancy Masquerading as an Incomplete Abortion- A Learning Lesson. J Clin Diagn Res. 2017;11(3):QD04-QD5.}
7. ^{Takeda A, Koike W, Hayashi S, Imoto S, Nakamura H. Cervico-isthmic pregnancy: early diagnostic imaging and successful dual therapy for uterine-sparing treatment. J Minim Invasive Gynecol. 2015;22(4):678-83.}
8. ^{Iloabachie GC, Igwegbe AO, Izuora KL. Cervico-isthmic twin pregnancy carried to 37 weeks. Int J Gynaecol Obstet. 1993;40(1):59-61.}
9. ^{Kayem G, Deis S, Estrade S, Haddad B. Conservative management of a near-term cervico-isthmic pregnancy, followed by a successful subsequent pregnancy: a case report. Fertil Steril. 2008;89(6):1826.e13-5.}
10. ^{Itakura A, Okamura M, Ohta T, Mizutani S. Conservative treatment of a second trimester cervicoisthmic pregnancy diagnosed by magnetic resonance imaging. Obstet Gynecol. 2003;101(5 Pt 2):1149-51.}
11. ^{Sakai A, Fujita Y, Yumoto Y, Fukushima K, Kobayashi H, Wake N. Successful management of cervico-isthmic pregnancy delivered at term. J Obstet Gynaecol Res. 2013;39(1):371-4.}

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