New HIV infections among children have declined significantly more slowly in West and central African countries (WCA) than in eastern and southern African countries between 2009 and 2015. Since adolescent fertility is particularly high in WCA countries, frequent mother-to-child transmission (MTCT) of HIV may in part be due to low coverage of HIV testing among adolescents during antenatal care (ANC). We investigated this adolescent gap in HIV testing using survey data from the demographic and health surveys and multiple indicators cluster surveys collected in 21 WCA countries since 2009. We found significant adolescent gaps in HIV testing in 12 out of 21 WCA countries, with the largest gap observed in Nigeria. In countries with a significant adolescent gap in HIV testing, we used Fairlie decompositions to assess what proportion of these gaps were explained by age-related differences in the distribution of (a) marital status, (b) socioeconomic status (SES), (c) MTCT-related knowledge, and (d) patterns of ANC utilization. Differences in SES and MTCT-related knowledge were the most consistent determinants of adolescent gaps in HIV testing during ANC. Differences in ANC utilization (e.g., fewer and possibly delayed ANC visits) also contributed to the adolescent gap in 8 out of 12 countries. Interventions that improve knowledge of MTCT risks, and/or promote the sustained use of ANC services, could help engage HIV-infected adolescents who become pregnant in PMTCT services. Targeting these interventions at the most disadvantaged households will be crucial in further reducing HIV infections among children.