1. 1. Mean sac diameter (MSD) (Fig. 2.21.1): It is the measurement of gestational sac which is usually seen 5 weeks after the last menstrual period or when it measures around 2–3 mm.
   • Technique: Assessment of gestational sac or mean sac diameter should preferably be done by transvaginal sonography (TVS). However, transabdominal ultrasound (TAS) may be used after 6 weeks of gestation. In an early intrauterine normal pregnancy, gestational sac appears as a discrete, small, well-circumscribed, round anechoic collection with bright echogenic edges embedded within endometrium.
     • Three mutually perpendicular measurements of the gestational sac diameter are averaged. The callipers are positioned at the inner edges of the gestational sac.

MSD = ( Length + height + width ) 3 ( Mean gestational sac diameter value ( in mm ) + 30 ) = Days of gestation

1. 2. Crown-rump length (CRL) measurement (Fig. 2.21.2): CRL is the length of fetus or embryo from top of head to bottom of torso. This primarily assists in assessment of gestational age.
   • Technique: Transvaginal scan is preferable on an empty bladder. Transabdominal sonography can also be done in the late first trimester and early second trimester with patient having adequately distended urinary bladder.
   • Image: Mid-sagittal image of the embryo.
     • Image should be magnified so that gestational sac should occupy majority of the screen. In early gestation, when crown and the rump cannot be convincingly identified, the maximum length of the embryo is measured. Subsequently, the maximum length between the crown and the rump should be measured. Care must be taken that callipers are placed at the distal-most margins with yolk sac and extremities not included in the measurement.
2. 3. Yolk sac: A normal yolk sac is identified as a round structure with an anechoic centre. Usually, it is well seen in a TVS study when mean gestational sac diameter is 5-6 mm (Fig. 2.21.3).
   • Technique: Measurement is preferably done on TVS with empty bladder. Calliper measures inner diameter from end to end after adequate magnification of the image.
   • Normal range: It is 3–5 mm. It usually gradually increases in size from 5th to 10th gestational week and later decreases in size.
     1. Large or small yolk sac – usually associated with poor obstetric outcome and are covered in Chapter 12.3: Ultrasound Assessment of the 5–11 Weeks of Pregnancy.
3. 4. Pregnancy failure including blighted ovum (Table 2.21.1): Diagnosing early first trimester failed pregnancy has always been a challenge, both for radiologists and the clinicians. Overtime the diagnostic criteria have evolved with inputs from both the radiology and obstetrics fraternity. The various sonographic findings have been labelled diagnostic for early pregnancy loss while others are considered suspicious and require further follow up. The authors recommend that transvaginal sonography (TVS) should be the preferred modality for evaluating pregnancy failure. The sensitivity of transabdominal ultrasound is relatively low in early pregnancy.
   • Blighted ovum refers to a gestational sac that forms and grows while an embryo fails to develop. On ultrasound examination, an empty gestational sac without an embryo is noted.
4. 5. Nuchal translucency (NT) measurement: It refers to a normal fluid-filled sonolucent subcutaneous space at the nape of the neck.
   • The Fetal Medicine Foundation has described a criteria for the measurement of NT (Fig. 2.21.4; Table 2.21.2). It is impressed that correct technique and accurate measurement of NT are vital for aneuploidy screening. The images can be further optimized by keeping angle of insonation 90 degrees (Fig. 2.21.5).
   • Historically, a value of less than 2.2 mm (95th percentile at 45 mm CRL) to 2.8 mm (95th percentile at 84 mm CRL) in thickness is considered as normal.
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