L utropin (LH) directs ovulation and implantation by regulating the production of estrogen and progesterone. We have shown that the circulatory half-life of LH is controlled by the Man/GalNAc-4-SO₄ receptor, which binds GalNAc-4-SO₄ on LH oligosaccharides. The short half-life in conjunction with episodic release of LH from the pituitary accounts for the pulsatile rise and fall in circulating LH. Complete genetic ablation of the Man/GalNAc-4-SO₄ receptor results in death in utero. Heterozygous female mice clear LH from the circulation more slowly and have smaller litters due to a reduction in the rate of implantation. This reduction is fully correctable by exogenous progesterone and estrogen, indicating that the rate of LH clearance is critical for the production of sufficient progesterone and estrogen for implantation. Thus, the Man/GalNAc-4-SO₄ receptor regulates the endocrinological status of the female and is essential for an early event in embryonic development.