Intended for healthcare professionals

Endgames Statistical Question

Analysis of longitudinal studies

BMJ 2013; 346 doi: (Published 18 January 2013) Cite this as: BMJ 2013;346:f363
  1. Philip Sedgwick, reader in medical statistics and medical education1,
  2. Louise Marston, research statistician2
  1. 1Centre for Medical and Healthcare Education, St George’s, University of London, Tooting, London, UK
  2. 2Department of Primary Care and Population Health, University College London, Royal Free Campus, London, UK
  1. p.sedgwick{at}

A randomised controlled trial investigated whether a low glycaemic index diet in pregnancy reduced the incidence of macrosomic (large for gestational age) infants. Participants were an at risk group—women without diabetes, in their second pregnancy, who had previously given birth to an infant weighing more than 4000 g. The intervention consisted of a low glycaemic index diet from early pregnancy; the control was no dietary intervention. The primary outcome measure was birth weight, and the secondary outcome was gestational weight gain from baseline (measured at 24, 28, 34, and 40 weeks). Treatment groups were compared at each gestational age using the independent samples t test.1

The researchers reported that a low glycaemic index diet in pregnancy did not reduce the incidence of large for gestational age infants in women at risk of fetal macrosomia. It did, however, have a significant effect on gestational weight gain. At each measured gestational age, except for 24 weeks, the intervention group gained significantly less weight than the control group.

View this table:

Comparison of gestational weight gain between intervention and control groups

Which of the following statements, if any, are true?

  • a) Measurements of gestational weight gain for each woman …

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