The aim of this work was to test whether postaxial hexadactyly had different clinical and epidemiological characteristics depending on hand or foot involvement. In the period 1967-1993, the Latin-American Collaborative Study of Congenital Malformations (ECLAMC) enrolled 1,582,289 births, and 2,271 cases with isolated (nonsyndromal) postaxial polydactyly (5th-digit hexadactyly). The prevalence was 14.3/10,000 births. Postaxial polydactyly (PP) of the hand (HPP) was the most frequent type (N:1,733; 76.3%; prevalence: 11.0/10,000), followed by foot PP (FPP=N:351; 15.5%; prevalence: 2.2), and hand and foot PP (BPP=N:187; 8.2%; prevalence: 1.2). Unlike HPP (55.0% bilateral; 77.2% left), FPP was less frequently bilateral (19.4%), with lower preference for the left side (55.5%). As expected, HPP was associated with African Black ethnicity, male sex, twinning, low maternal education, parental consanguinity, and there was frequent recurrence in 1st-degree relatives. Conversely, FPP was associated with Amerindian racial background, parental subfertility, and bleeding in the 1st trimester of pregnancy. BPP displayed the highest frequency of associated congenital defects (23.4%, vs HPP:6.6%, FPP: 15.4%). In its isolated form, BPP resembled HPP more than FPP with respect to left preference (90.9%), familial recurrence (11.0% of 1st degree relatives), and low maternal education. Although male sex preference and high frequency of twinning was observed in the 3 PP subtypes, statistical significance was present only in HPP. None of the 3 PP subtypes showed abnormal values for perinatal mortality, birth weight, length of gestation, parental ages, or parity. A logistic regression analysis showed Black race only to be associated with HPP, parental subfertility with FPP, parental consanguinity with BPP, and non-Black race with both FPP and BPP. The data presented here are the first indication that HPP and FPP are 2 different entities, with a larger genetic component in HPP than in FPP.

This work includes all cases with extra digits (polydactyly) registered from a birth sample of over four million births aggregated from two comparable birth series: the Latin‐American Collaborative Study of Congenital Malformations: ECLAMC (3,128,957 live and still births from the 1967 to 1993 period), and the Spanish Collaborative Study of Congenital Malformations: ECEMC (1,093,865 livebirths from April 1976 to September 1993, and 7,271 stillbirths from January 1980 to September 1993). All but 2 of 6,912 registered polydactyly cases fit well into one of the following 11 pre‐established polydactyly types (observed number of cases in parentheses): Postaxial hexadactyly (5,345), Preaxial‐I hexadactyly (1,018), Seven or more digits (57), synpolydactyly (15), crossed polydactyly (45), 1st digit triphalangism (33), 2nd digit duplication (39), 3rd digit duplication (18), 4th digit duplication (22), Haas polysyndactyly (3), and high degree of duplication (4). The birth prevalence rates observed in both series were similar except for postaxial polydactyly, which was more frequent in the ECLAMC (150.2/100,000) than in the ECEMC (67.4/100,000), as expected due to the higher African Black ethnic extraction of the South‐American than of the Spanish populations. This similar frequency for the rare polydactylies (5.4 per 100,000 in South America and 5.7 in Spain), and for each one of the 9 categories, suggests that the values reported here are valid for most populations. The rare polydactylies are frequently syndromal: one third of them (77/236) were found in association with other congenital anomalies, 11.0% (26/236) in MCA cases and 21.6% (51/236) in recognized syndromes. © 1996 Wiley‐Liss, Inc.