Cornual pregnancy results from the implantation of the pregnancy in an atretic functional noncommunicating rudimentary cornu in women with unicornuate uteri. It is a rare form of ectopic pregnancy occurring in 1/76000 of all pregnancies [58].

Similar to interstitial pregnancy, the pregnancy can follow a silent presentation in the early weeks until uterine rupture and major internal bleeding occur, which typically occurs in the second trimester in about 50 % of cases. The typical sonographic features include an empty uterine cavity with a single interstitial tube. The gestational sac is separate from the main uterine corpus and surrounded by a thick wall representing the myometrium of the rudimentary cornu. A thick vascular pedicle connecting the empty corny and the gestational sac is another helpful sign [37, 51].

This occurs due to the implantation of the pregnancy in the peritoneal cavity outside the uterine cavity and the fallopian tube and ovaries. It happens in 1 % of all ectopic pregnancies and is related to high morbidity and mortality when compared to other types of ectopic pregnancy [7, 17].

Two theories behind this type of pregnancy have been described: the first type happens when the implantation of the pregnancy happens in the peritoneal cavity, and this is called a primary abdominal pregnancy, while a secondary abdominal pregnancy, which is more common, is the outcome of tubal miscarriage and reimplantation of the pregnancy in the abdominal cavity. Broad ligament with its high vascularity provides a good support for such type of pregnancy and thus is a common location for the implantation [7
, 17] (Fig. 13.9).

The ultrasound diagnosis includes empty uterine cavity and the identification of pregnancy sac separate from the pelvic organs. The ultrasound findings of a normal uterus with two interstitial portions of the tube help to differentiate between abdominal and cornual pregnancies. In advanced abdominal pregnancy, there is often severe foetal growth retardation and oligohydramnios [2, 8, 49].

This pregnancy is the result of pregnancy implanting in the ovarian substance or on the surface of the ovary and accounts for almost 3 % of all ectopic pregnancies [34
]. The clinical presentation is usually similar to tubal ectopic pregnancy, but the ultrasound morphology differs, and it includes the presence of a gestational sac surrounded by ovarian tissue and separate from the corpus luteum, which can be found in the ipsilateral or contralateral ovary. In addition there will be a negative sliding sign when trying to separate the gestational sac from the ovary on gentle palpation. A correct ultrasound diagnosis is not always possible especially when there is pelvic adhesions causing a tubal ectopic pregnancy to be adherent to the ovary and mimicking ovarian pregnancy [21, 61] (Fig. 13.10).

Both of these ectopic pregnancies share the fact that they are the results of previous surgical trauma to the uterus. They are both characterised by trophoblast invasion beyond endometrial-myometrial junction, and they are classified as intramural pregnancies. Due to myometrial involvement, heavy bleeding often complicates surgical evacuation or even spontaneous expulsion [15].

Abnormal, painless vaginal bleeding is the main clinical presentation in these types of pregnancies. However, abdominal pain and even urinary symptoms caused by trophoblastic invasion into the bladder can occur in more advanced cases. Cervical pregnancy tends to be located low in the cervix in contrast to Caesarean scar pregnancy that is typically present close to the internal os [63, 73].

Correct ultrasound diagnosis is essential to differentiate between these types of ectopic pregnancies. It is even more important to differentiate them from a normal intrauterine pregnancy or inevitable miscarriage in order to avoid serious complications. The first ultrasound criterion for diagnosis is identifying the gestational sac or trophoblastic tissue at or below the level of the internal os with a clear invasion into the myometrium. Sliding organ sign will be negative as the gestational sac cannot be displaced from its surrounding. Another important ultrasound feature is the high blood flow (>20 cm/s) in the peritrophoblastic circulation on colour Doppler examination. This is different to inevitable miscarriage where the gestational sac usually changes position on applying gentle pressure with the ultrasound probe with no evidence of trophoblastic blood flow in the nearby myometrium [38, 60
,
74


] (Figs. 13.11, 13.12, 13.13, 13.14 and 13.15).

The other differential diagnosis is low normal intrauterine pregnancy where the challenge will lie in differentiating the exact position of the pregnancy especially if it was very near to the internal os. This becomes even harder when the pregnancy is more advanced than the first trimester. However, the identification of the uterine artery insertion can be a useful guide to the level of the internal os, and the identification of myometrium invasion by trophoblastic tissue is usually the key factor in establishing the correct diagnosis [39, 72, 73].