Table 1.

Description of mediator variables

Variables	Description
	DNBC	MCS
Perinatal factors
Smoking in pregnancy	Mothers were asked at, on average, 16 to 17 weeks of gestational age whether they smoked during pregnancy (yes/no)	Mothers were asked when the child was 9 months old whether they smoked before pregnancy (yes/no), and whether they changed after becoming pregnant (yes/no). Those who did not give up smoking during pregnancy were considered having smoked during pregnancy
Alcohol use in pregnancy	Mothers were asked at, on average, 16 to 17 weeks of gestational age about the number of units of alcoholic beverage namely beer, wine and spirit, that the mothers drank per week. Amount of alcohol consumption was categorised as: (1) did not drink alcohol during pregnancy; (2) light drinker: one to two units per week; (3) moderate drinker: three to six units per week; and (4) heavy drinker: seven units or more.41 However, only a few observations falls into the category of heavy drinker (n<10); we collapsed the heavy drinker category into moderate drinker. A unit of alcohol was defined as one bottle of beer, one glass of wine, or one glass of spirits (about 4 cL), each of which corresponds to about 12 g of alcohol42	Mothers were asked when the child was 9 months old if they drank alcohol during pregnancy, the number of units they consumed per week. Amount of alcohol consumption was categorised as: (1) did not drink alcohol during pregnancy; (2) light drinker: one to two units per week; (3) moderate drinker: three to six units per week; and (4) heavy drinker: seven units or more.41 The heavy drinker category was collapsed into moderate drinker category as some cells only had small number of observations (n~10) when cross-tabulated with maternal education. A unit of alcohol was defined as approximately half a pint of beer or one glass of wine, which is around 10 g of alcohol43
Birth weight	Data on birth weight in grams were obtained by data linkage to the Danish medical birth registry of Denmark	Mothers were asked when the child was 9 months old about the birth weight of their child, in kilograms or pounds. Birth weights were then converted to grams for analysis
Gestational age	Data on gestational age in days were obtained by data linkage to the Danish medical birth registry of Denmark	Gestational age in days was calculated on the basis of the mother’s report of her expected due date44
Childhood illness at age 7 years	Mothers were asked whether the child had any handicap or chronic illness (yes/no)	Mothers were asked whether the child had any longstanding illness/disability/infirmity (yes/no)
Maternal mental health at age 7 years	Mothers were asked whether she had a psychiatric illness/bad nerve since birth (yes/no)	Maternal psychological distress was assessed using Kessler-6 scale,45 asking whether in the last month how often respondents felt depressed, hopeless, restless or fidgety, worthless or that everything was an effort. Validated cut-off scores were used: normal (0 to 5); distress (6 to 24)

DNBC, Danish National Birth Cohort; MCS, Millennium Cohort Study.