Context: Serum human chorionic gonadotropin (hCG) levels are essential for diagnosing and monitoring early pregnancy. Obesity is a health care epidemic; however, the performance of this vital serum hormone in women with an elevated body mass index (BMI) is unknown.
Objective: To investigate the association of BMI with serum hCG values and rate of hCG increase.
Design: Retrospective cohort study.
Setting: University-based infertility clinic.
Patients: Women undergoing fresh vs frozen in vitro fertilization cycles with single-day three or five embryo transfers resulting in singleton live births (≥24 weeks' gestational age) from 2008 to 2015.
Intervention(s): None.
Main outcome measure(s): The initial hCG (mIU/mL, 16 days after oocyte retrieval) and 2-day percentage of hCG increases among BMI categories were compared using multivariable linear and logistic regression, adjusted a priori for the day of embryo transfer.
Results: The initial serum hCG values correlated inversely with the BMI (P < 0.0001, test for trend). Low initial hCG values (<100 mIU/mL) were significantly more common across increasing BMI classes, from 1.4% of normal weight patients to 15.6% of those with a BMI ≥40 kg/m2 (P = 0.001, test for trend). The mean 2-day hCG increases were similar and normal (≥53%) across the BMI groups.
Conclusions: Patients with obesity achieving live births had statistically significantly lower initial serum hCG values compared with patients who were nonobese. However, the mean 2-day percentage of increases in hCG were similar across BMI categories. The initial hCG values might lack sensitivity for live births in patients with obesity. The rate of hCG increase remains the mainstay of monitoring very early pregnancies after in vitro fertilization. Future studies should investigate whether serum analyte ranges should be adjusted according to the BMI.