Imaging description

The appearance of early pregnancy on ultrasound is well known, but the CT and MRI findings are less well described. In the first trimester, pregnancy results in an endometrial cyst-like structure (the gestational sac) of variable size at CT or MRI (Figures 80.1–80.6) [1,2]. Fetal parts are usually not visible until the late first trimester. The developing placenta may be seen as a peripheral curvilinear enhancing structure. Later in gestation, the diagnosis of pregnancy is straightforward as the fetus becomes obviously visible. The gestational sac of early pregnancy may be small and inconspicuous at CT and MRI, but the co-existence of an ovarian corpus luteum cyst can be an important clue to the diagnosis of pregnancy (Figures 80.1 and 80.3–80.5). Remember the corpus luteum (which normally regresses in the second half of the menstrual cycle) persists as a critical source of progesterone in early pregnancy. A corpus luteum can nearly always be identified between five and eight weeks' gestation, but after that the corpus luteum shrinks (as the placenta becomes the source of progesterone in later pregnancy) and may not be visible at imaging [3–5].

Importance

It is important to know the appearance of early pregnancy on CT or MRI because occasionally the patient is unaware of being pregnant and the radiologist is the first to make the diagnosis. Failure to consider the possibility of pregnancy for an endometrial cyst-like structure might result in unnecessary workup, or even repeat irradiation. With respect to the latter, it should be remembered that the radiation dose to the fetus from a diagnostic abdominopelvic CT scan rarely merits consideration of pregnancy termination [6, 7].