A genetically predicted elevation in the relative count of circulating CD28−CD25++CD8+ T cells was causally related to increased female infertility risk (OR [95% CI], 1.26 [1.14–1.40], p = 1.07 × 10−5), whereas a high absolute count of NKT cells reduced the risk of ectopic pregnancy (OR [95% CI], 0.87 [0.82–0.93], p = 5.94 × 10−6). This evidence concerns the gene CD8A and female infertility.