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

EndpointDefinition caseDefinition controlNo of mother–child pairs included in the regression *Adjustment in stratified multiple logistic regression
Quit during first trimester †Continued after first trimester ‡
 Preterm birthGestational age <37 weeksGestational age ≥37 weeks1 210 4101 286 667Maternal age, sex, parity, SES
 Low birth weightBirth weight <2500 gBirth weight 2500–4500 g1 170 1871 328 221Maternal 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) birth1 210 0481 327 783Maternal age, sex, parity, SES
 Body length<10th percentile of length for gestational age (weeks) birth≥10th percentile of length for gestational age (weeks) birth1 210 0481 327 783Maternal 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) birth573 343601 407Maternal age, sex, parity, SES
Abnormal body proportions
 Ponderal index>90th percentile of weight-to-length ratio for gestational age (weeks) birth10-90th percentile of weight-to-length ratio for gestational age (weeks) birth1 088 4511 196 479Maternal age, sex, parity, SES, weight z-score
 Brain-to-body ratio §<10th percentile of weight-to-head circumference ratio for gestational age (weeks) birth10-90th percentile of weight-to-head circumference ratio for gestational age (weeks) birth518 704541 549Maternal age, sex, parity, SES, weight z-score
 Head-to-length ratio §>90th percentile of head circumference-to-length ratio for gestational age (weeks) birth10-90th percentile of head circumference-to-length ratio for gestational age (weeks) birth521 420547 597Maternal 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.