The measurement of human T-cell leukemia virus type 1 (HTLV-1) proviral DNA levels by using polymerase chain reaction has been beneficial for confirming HTLV-1 infection during pregnancy. However, the influence of pregnancy on HTLV-1 infection and proviral DNA levels among pregnant women with HTLV-1 has not been clarified. We prospectively gathered blood samples from 36 pregnant women in whom HTLV-1 carriage was previously diagnosed and sequentially measured their proviral DNA levels. The HTLV-1 proviral DNA levels remained at a plateau during pregnancy but were elevated after delivery. Moreover, flow cytometry and serological analyses revealed that the regulatory T-cell population and soluble interleukin 2 receptor levels were similarly elevated after birth in comparison with those in control pregnant women. This study is the first to provide data on sequential changes in HTLV-1 proviral DNA levels during and after pregnancy. These findings will guide the establishment of a better program to prevent mother-to-child transmission of HTLV-1.