Table 1 When evaluating different ethnic populations, a negative correlation was found between MTHFR 677TT homozygosity and the incidence of spontaneously conceived dichorionic twins: a relatively high number of MTHFR 677TT homozygous individuals (20 %) are reported in Asian populations where dichorionic twins are relatively rare (6 out of 1 000). In African populations, the incidence of spontaneously conceived dichorionic twins is relatively high at 30 per 1 000, while the incidence of MTHFR 677TT homozygosity is below detection limits. The Caucasian population holds a mid-position, with 11–13 spontaneously conceived twin births out of 1 000 and a MTHFR 677TT homozygosity of around 9 % (modified from 23).

	Incidence of twins (1/1 000)	TT homozygosity (%)
African populations Ghana West Africa	32.1	0.0
Caucasian populations Netherlands Central Europe	11.0–13.0	9.0
Asian populations Hong Kong South China	6.0	20.0