Table 1.

 Characteristics of twins and singletons born in Denmark during 1986-8. Values are numbers (percentages) unless stated otherwise

Characteristics	Singletons*	Twins
No of live births	8280	3652
Deaths (days after birth):	90 (1.1)	97 (2.7)
1-28	34 (0.4)	72 (2.0)
≥29†	56 (0.7)	25 (0.7)
Emigrated†	394 (4.8)	144 (3.9)
Study base‡	7796 (94.2)	3411 (93.4)
Mean (SD) median No of days in hospital (age <1 year)	6.6 (9.2) 5	16.0 (18.4) 10
Mean (SD) median No of days in hospital (age 1-14 years)	3.5 (14.1) 0	3.6 (12.5) 1
Hospital stay (days) age 1-14 years:
None	4064 (52.1)	1673 (49.0)
1-2	1730 (22.2)	827 (24.2)
3-30	1863 (23.9)	841 (24.7)
31-60	86 (1.1)	46 (1.3)
≥61	53 (0.7)	24 (0.7)
Male	4030 (51.7)	1715 (50.3)
Mean (SD) birth weight	3449 (537); n=7771	2541 (547); n=3406
Mean (SD) gestational age	39.7 (1.7); n=7759	37.0 (2.5); n=3406
Mean (SD) maternal age	27.8 (4.8); n=7795	29.0 (4.6); n=3411
Mean (SD) paternal age	30.8 (5.8); n=7738	31.9 (5.7); n=3402
Mean (SD) maternal education§	1.67 (1.72); n=7629	1.72 (1.77); n=3318
Mean (SD) paternal education§	1.58 (1.76); n=7330	1.68 (1.83); n=3233
School performance:
Test score available	6575 (84.3)	2866 (84.0)
Mean (SD) test score	8.02 (1.06)	8.02 (1.05)
Mean (SD) age at test	16.0 (0.3)	16.0 (0.3)
Teacher score available	6572 (84.3)	2851 (83.6)
Mean (SD) teacher score	8.05 (1.08)	8.09 (1.06)
Mean (SD) age at test	16.0 (0.3)	16.0 (0.3)
Information on all variables	6018 (77.2)	2611 (76.5)

Denominators vary owing to missing values.

*5% random sample of Danish birth cohorts, 1986-8.

†Before 1 January 2003.

‡Rest of table gives percentage of study base.

§0=basic school 8th-10th form; 1=vocational main course; 2=upper secondary education; 3=short cycle higher education; 4=medium cycle higher education; 5=bachelors degree; 6=masters degree and PhD.