Estimated fetal weight

TTTS. Estimated fetal weight and fetal weight discordance

The legacy of neonatal criteria for the diagnosis of TTTS still lingers. A 20% discordance in birthweight was to establish the diagnosis of TTTS. Prenatal ultrasound diagnosis based on specific amniotic fluid discordances, as opposed to postnatal criteria, has eliminated birthweight discordance as a criterion. Figure 7.16 shows the frequency distribution of EFW discordance as determined by ultrasound. 

As can be seen, the spectrum ranges from 0 to 65%, approximately. Restriction of the definition of TTTS to only those fetuses with at least 20% discordance would disqualify approximately 31% of all TTTS patients. The reasons for the wide range of EFW discordances are not entirely clear. Intrauterine growth retardation (IUGR) may exist independently of TTTS, as a completely separate entity. 

We now know that growth restriction may occur as a result from lack of individual placental territory, from the placental vascular anastomoses themselves, or from both. The contribution of either of these factors, either independently or in conjunction, cannot be predicted prenatally. 

Although EFW discordance may provide information about the disparity in growth, a more useful parameter to establish is the actual percentile growth for each fetus, particularly if it is below the 10th percentile. Table 7.1 shows the cut-off values used in our laboratory to diagnose IUGR (estimated fetal weight <10th percentile). 

TTTS patients in whom one or both twins are classified as having IUGR should then be labeled as having both diagnoses, i.e. TTTS and IUGR. This approach may facilitate the communication between investigators and the counseling of patients for treatment. Our data show that approximately 50% of TTTS patients have IUGR of one of the fetuses.