The East Flanders Prospective Twin Survey (EFPTS).

The East Flanders Prospective Twin Survey (EFPTS) is a prospective, population-based registry of multiple births in the province of East Flanders, Belgium. EFPTS has several unique features: it is population based and prospective, with the possibility of long-term follow-up; the twins (and higher order multiple births) are ascertained at birth; basic perinatal data recorded; chorion type and zygosity established; and since 1969 placental biopsies have been taken and frozen at -20 degrees C for later determination of genetic markers. The EFPTS is the only large register that includes placental data and allows differentiation of 3 subtypes of monozygotic (MZ) twins based on the time of the initial zygotic division: the dichorionic-diamnionic pairs (early, before the 4th day after fertilization), the monochorionic-diamnionic pairs (intermediate, between the 4th and the 7th day post fertilization), and the monochorionic-monoamnionic pairs (late, after the 8-day post fertilization).