Twins

Approximately 4 per 1000 births worldwide are monozygotic
Approximately 2/3 of twins are dizygotic, but incidence varies by race
- 10-40 per 1000 in blacks
- 7-10 per 1000 in whites
Only 6-10% of artificially conceived twins are monozygotic
Since 1970 the prevalence of multiple births has been increasing
- From 2004-2006 the prevalence has stabalized in the US at 32 per 1000 births
The Yorubas of western Nigeria have a birth rate of 45 twins per 1000 live births, of which approximately 90% are dizygotic (higher incidence is often attributed to the nutrients they get from a type of yam they eat)
"Approximately 30% of monozygotic twins have dichorionic/diamniotic placentas."
"Monochorionic/monoamniotic twins are rare; only 1% of monozygotic twins have this form of placentation."

(Most of these statistics are from Medscape.com)

	Twins	Triplets	Quadruplets	Quintuplets and higher	Singletons
Number of births	137085	6118	355	67	4121930
% of total births	3.213	0.143	0.008	0.001	96.632

% less than 32 weeks Gestation	12.1	36.3	79.2	79.1	1.6
% less than 37 weeks of Gestation	60.4	92.6	94.9	89.6	11.1

Mean gestational age in weeks (std dev)	35.2 (3.6)	32.0 (3.9)	29.3 (4.1)	29.4 (5.3)	38.7 (2.4)

% birthweight under 1500 grams	10.2	34.8	73.4	84.8	1.1
% birthweight under 2500 grams	57.5	95.4	98	95.5	6.5

Mean birthweight in grams (std dev)	2,323 (629)	1,655 (557)	1,255 (543)	1,147 (641)	3,298 (565)