A thickened nuchal fold was the first described sono­graphic sign of DS in 1985 by Benacerraf et al. [7]. From then on, many sonographically identified markers associ­ated with DS, have been described [2,8-11]. They include short femur, short humerus, increased nuchal fold thicken­ing, pyelectasis, echogenic bowel, choroids plexus cysts, hypoplastic middle phalanx of the fifth digit, wide space between the first and second toes (sandal gap), a two-ves­sel umbilical cord and many others which are  presented in Table 1. Most fetuses with trisomy 21 will have more than one ultrasound marker present [2,8-10]. Moreover, the presence of multiple ultrasound markers is the rule rather than the exception in fetuses with trisomy 21. Thus, the question arises as to which markers are the most diag­nostically efficient.

Trisomy 18. Also called the Edward syndrome, has an incidence of 3/1,000 births [12]. It is associated with se­vere growth restriction and multiple structural malforma­tions that involve practically all organs [13-15] (Table 1). By sonography, about 80-85% of fetuses with trisomy 18 are detected prenatally [15]. Major cardiac abnormalities are present in more than 90% of the affected fetuses [12]. The detection of trisomy 18 by ultrasonography improves with gestational age, as severe growth restriction and poly­hydramnios become apparent. The majority of fetuses with trisomy 18 die in utero, probably because of placental insufficiency; those that survive until birth, die within a few hours, days or weeks after delivery.

Trisomy 13. This autosomal trisomy (also called Patau syndrome) has an incidence of 1/5,000 births [12]. The newborns die shortly, within days or weeks after de­livery. The associated sonographic abnormalities (Table 1) are obvious and severe, and most of them are detectable early in the second trimester. Throughout pregnancy, 90-100% of cases are sonographically detectable [13,16]. Almost all the systems are involved, mainly the central nervous system (holoprosencephaly, midline facial, and neural tube defects), the heart (defects in more than 90% of the fetuses), the kidneys (polycystic kidneys in 30% of cases) [13,16,17]. A differential diagnosis with Meckel-Gruber syndrome is sometimes needed because of the same pattern of abnormalities. This lethal autosomal reces­sive condition has a risk of recurrence of 25% in the same family.

Triploidy. This is the result of three complete sets of chromosomes. In most cases, the extra set of chromosomes is paternally derived. The vast majority of triploidy con­ceptions are lost early in gestation, if however, there is a double maternal chromosome contribution, the pregnancy may persist into the third trimester [1,12]. In ultrasonog­raphy, these fetuses have multiple severe malformations, and severe asymmetrical restriction of early onset [18]. In 60-90% of cases, the placenta initially appears to be of a normal consistency, but progressively becomes large, hydropic-appearing, which is consistent with partial moles. This occurs when the third set of chromosomes is paternal in origin [12]. When the excess set of chromosomes is maternally derived, the placenta is small and associated with oligohydramnios. The most common sonographic features are summarized in Table 1. In triploidy, as in tri­somy 18, the correct antenatal diagnosis is very important, since the affected fetuses may be mistaken for normal fetuses with growth restriction and placental insufficiency, which would stimulate aggressive management such as operative delivery, with no benefit since the outcome is extremely poor.

Turner Syndrome. The non lethal type usually does not demonstrate any ultrasonographic abnormalities. In the lethal type, intrauterine lethality between 12-40 weeks occurs in 75% of the fetuses [1].These fetuses present in the sonogram with large nuchal cystic hygromata, general­ized edema, mild pleural effusions and ascites, and car­diac abnormalities (Table 1).

Sex Chromosome Abnormalities. Sex chromosome abnormalities other than Turner syndrome, include the 47,XXX, 47,XXY, and 47,XYY. They are not associated with an increased prevalence of sonographically detect­able defects [1].