title: Association between efficacy and body weight or body mass index for two low-dose oral contraceptives.
abstract: BACKGROUND
  This analysis investigated the association of oral contraceptive efficacy with body weight and body mass index (BMI) for hypothesis-generating purposes.
  
  
  METHODS
  Data were from a randomized, parallel-group trial of 180/215/250 mcg of norgestimate (NGM)/25 mcg of ethinyl estradiol (EE) (given to 1671 women) and 1 mg of norethindrone acetate (NETA)/20 mcg of EE (given to 1139 women). Pregnancies were evaluated across BMI deciles and by BMI and body weight dichotomies. A Pearl index was calculated for each treatment group. The relative risk (RR) of pregnancy was calculated with a Cox proportional hazards model.
  
  
  RESULTS
  The Pearl index for women who received NGM/EE was 2.36 [95% confidence interval (CI)=1.33-3.40]; for those who received NETA/EE, the Pearl index was 3.29 (95% CI=1.81-4.77). Consistent, weak positive associations between weight and pregnancy risk were found. Overall, for women with a BMI >or=25 kg/m(2) (compared with women with a BMI <25 kg/m(2)), the RR of pregnancy was 1.84 (95% CI=0.98-3.45); that for women who received NGM/EE was 1.39 (95% CI=0.57-3.40), whereas that for women who received NETA/EE was 2.49 (95% CI=1.01-6.13). For women with a body weight >or=70 kg (compared with women with a body weight <70 kg), the RR was 1.25 (95% CI=0.63-2.46); that for women who received NGM/EE was 1.41 (95% CI=0.56-3.54), whereas that for women who received NETA/EE was 1.12 (95% CI=0.40-3.12).
  
  
  CONCLUSIONS
  Women in the higher body weight or BMI category showed a small increase in the risk of pregnancy with these oral contraceptives, but this increase was not statistically significant overall or for either formulation studied.
authors.affiliation: Department of Obstetrics and Gynecology, Baystate Medical Center, Springfield, MA 01199, USA. rtb@bhs.org
authors.forename: Ronald T
authors.initials: RT
authors.lastname: Burkman
authors.forename: Alan C
authors.initials: AC
authors.lastname: Fisher
authors.forename: George J
authors.initials: GJ
authors.lastname: Wan
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Barnowski
authors.forename: Katherine D
authors.initials: KD
authors.lastname: LaGuardia
mesh_terms: D000293:Adolescent; D000328:Adult; D015992:Body Mass Index; D001835:Body Weight / Q000502:physiology*; D003276:Contraceptives, Oral / Q000008:administration & dosage*; D004997:Ethinyl Estradiol / Q000008:administration & dosage; D005260:Female; D006801:Humans; D008875:Middle Aged; D009640:Norethindrone / Q000008:administration & dosage / Q000031:analogs & derivatives; D000077563:Norethindrone Acetate; D009644:Norgestrel / Q000008:administration & dosage / Q000031:analogs & derivatives; D011247:Pregnancy; D049168:Pregnancy, Unplanned; D016016:Proportional Hazards Models; D012189:Retrospective Studies; D012306:Risk; D055815:Young Adult
chemical_list: D003276:Contraceptives, Oral; D009644:Norgestrel; D004997:Ethinyl Estradiol; D000077563:Norethindrone Acetate; C017576:norgestimate; D009640:Norethindrone