Table 4

Estimates of effects of quitting smoking on log (birth weight)*

VariableMean (g)95% CIPr(>|t|)
Intercept: baseline (40 weeks)3233.13206.7 to 3259.8<0.001
Gestational age at birth, centred at 40 weeks
Each additional week past 40156.8151.0 to 162.7<0.001
Squared−12.6−13.2 to −12.4<0.001
Smoking status at delivery compared with smoker throughout
Non-smoker259.6241.8 to 277.7<0.001
Quitter210.2186.3 to 235.0<0.001
Parity compared with second child
First child−104.4−115.4 to −92.9<0.001
Third or more22.37.7 to 37.2<0.005
Ethnicity compared with  white
Caucasian−141.6−159.4 to −123.5<0.001
SEP (based on IMD score) compared with middle three-fifths
Least deprived fifth27.814.4 to 41.1<0.001
Most deprived fifth−15.5−40.4 to 10.10.232
BMI compared with recommended
Underweight (<20)−116.7−146.5 to −86.4<0.001
Overweight (25–29.9)79.565.7 to 93.2<0.001
Obese (30+)134.5119.3 to 150.3<0.001
Sex of baby compared with female
Male129.0117.0 to 141.5<0.001
  • 22 826 valid cases included in model (39.5% cases with missing data excluded).

  • *Estimates are backtransformed to represent the actual change in birth weight of babies with 95% CI and statistical significance.

  • BMI, boy mass index; SEP, socioeconomic position (categories defined by fifths of Index of Multiple Deprivation (IMD) score).