Table 1.

Summary of the definitions of endpoints and adjustments in the regression model used to study the association of maternal smoking and preterm birth, low birth weight, small for gestational age and abnormal body proportions at birth

Endpoint	Definition case	Definition control	No of mother–child pairs included in the regression *		Adjustment in stratified multiple logistic regression
			Quit during first trimester †	Continued after first trimester ‡
Preterm birth	Gestational age <37 weeks	Gestational age ≥37 weeks	1 210 410	1 286 667	Maternal age, sex, parity, SES
						Low birth weight	Birth weight <2500 g	Birth weight 2500–4500 g	1 170 187	1 328 221	Maternal age, sex, parity, gestational weeks, SES
Small for gestational age
Weight	<10th percentile of weight for gestational age (weeks) birth	≥10th percentile of weight for gestational age (weeks) birth	1 210 048	1 327 783	Maternal age, sex, parity, SES
						Body length	<10th percentile of length for gestational age (weeks) birth	≥10th percentile of length for gestational age (weeks) birth	1 210 048	1 327 783	Maternal age, sex, parity, SES
Head circumference	<10th percentile of head circumference for gestational age (weeks) birth	≥10th percentile of head circumference for gestational age (weeks) birth	573 343	601 407	Maternal age, sex, parity, SES
						Abnormal body proportions
Ponderal index	>90th percentile of weight-to-length ratio for gestational age (weeks) birth	10-90th percentile of weight-to-length ratio for gestational age (weeks) birth	1 088 451	1 196 479	Maternal age, sex, parity, SES, weight z-score
						Brain-to-body ratio §	<10th percentile of weight-to-head circumference ratio for gestational age (weeks) birth	10-90th percentile of weight-to-head circumference ratio for gestational age (weeks) birth	518 704	541 549	Maternal age, sex, parity, SES, weight z-score
Head-to-length ratio §	>90th percentile of head circumference-to-length ratio for gestational age (weeks) birth	10-90th percentile of head circumference-to-length ratio for gestational age (weeks) birth	521 420	547 597	Maternal age, sex, parity, SES

*The number of mother–child pairs is the sum of non-exposed and exposed pregnancies; the number varies for outcomes due to exclusion of mother–child pairs with missing information on the exposure, outcome or any of the confounding factors included in the multivariate regression.

†Comparison quit smoking during the first trimester with no smoking during pregnancy.

‡Comparison continued smoking after the first trimester with no smoking during pregnancy.

§Available for the years 2004–2016.

SES, socioeconomic status.