Case-control studies: sources of bias

Researchers explored the association between maternal intake of folic acid supplements, multivitamins, and dietary folates in the prevention of cleft lip (with or without cleft palate) in the mothers’ babies. A population based case-control study was performed. All babies with cleft lip referred for corrective surgery in Norway between 1996 and 2001 were identified. For each case—a baby with a cleft lip—a maximum of two live births without facial clefts were randomly selected from the Norwegian medical birth registry for 1996 to 2001 to serve as controls. Controls were matched to each case by date of birth to within six weeks of delivery.1

The mothers of all cases and controls were invited to take part in the study. If women agreed, they completed questionnaires several months after delivery, providing demographic characteristics, reproductive history, and details about smoking, alcohol and drug consumption, and other exposures during early pregnancy. Women were also asked to report whether they took folic acid supplements and, if so, the dosage consumed for each of the six months preceding pregnancy and each of the first three months of pregnancy. Similar information was also provided about …