Ultrasound still forms the mainstay of obstetrical imaging. It can be used throughout the entire gestation, resulting in high-resolution real-time two-dimensional (2D) images that can be obtained in any plane. This can be complemented by static three-dimensional (3D) images and motion-enabled four-dimensional (4D) images of the heart and other cardiovascular structures. Ultrasound may be complemented by MRI in the late second or third trimester for evaluating specific fetal or maternal abnormalities.

Indications for obstetric ultrasound are summarized in Table 17.1. The nuchal translucency scan is performed at 11–14 weeks and in the absence of specific indications, the mid-trimester scan should be obtained between 18 and 20 weeks of gestation, when anatomically complex organs such as the heart and brain can be imaged with sufficient clarity to allow detection of malformations.

First trimester

The gestational sac can be identified as early as 4 weeks 3 days and can be routinely detected endovaginally after 5 weeks. There is an echogenic rim of tissue surrounding the gestational sac (Fig. 17.1) comprising the:

• decidua basalis (DB) = beneath the implanted embryo; it is here that the placenta subsequently develops

• decidua capsularis (DC) = covers the rest of the chorionic sac

• decidua parietalis (DP) = endometrial reaction which lines the uterine cavity (UC) and is not involved in implantation.