First-trimester endocrinology is determined on the one hand by corpus luteum progesterone and estradiol biosynthesis and secretion and on the other hand by the time of onset and extent of progesterone and estradiol secretion by the placenta. This dual interplay is dominated by the corpus luteum placental shift, where a relative progesterone and estradiol deficiency can develop and may lead to early as well as late abortion. Measurements of serum progesterone and estradiol are mandatory in order to reveal such deficiencies and to treat them effectively with 17 alpha-hydroxyprogesterone caproate in combination with estradiol valerate or with progestins such as dydrogesterone.