Hormones such as progesterone, produced by the corpus luteum, the leftover "shell" of the follicle that contained the ovulated egg, play a crucial role in maintaining a pregnancy. One sign of inadequate progesterone levels is a short luteal phase, or luteal phase defect, after ovulation. Clomiphene citrate, a fertility drug commercially sold as Clomid, can help decrease luteal phase defect and raise progesterone levels in some cases.
Progesterone changes the composition of the uterine lining, increasing the number of blood vessels so an embryo can implant. Progesterone levels in the first half of the menstrual cycle remain low. After ovulation, as the corpus luteum produces progesterone, levels should rise to 15 ng/mL or higher. Progesterone levels normally remain high for around 12 days after ovulation. When progesterone levels fall, the uterine lining breaks down and your menstrual period starts. If you have a luteal phase defect, the corpus luteum doesn't produce enough progesterone to maintain the lining and your period starts 10 days or less after ovulation.
Clomid, normally taken for five days at the beginning of the menstrual cycle, acts as both an estrogenic and anti-estrogenic drug. Clomid occupies estrogen receptors on the ovary, pituitary, hypothalamus and endometrial tissue. Clomid stimulates the release of gonadotropins, hormones that stimulate follicle development and egg maturation. Clomid not only induces ovulation in women who may not ovulate at all but also produces a "better" follicle. A more mature and well-developed follicle will produce a better corpus luteum, which may in turn produce more progesterone.
Clomid does not increase progesterone levels on its own; only if the drug stimulates production of a more mature follicle will it have any effect on progesterone levels. Some women will still need supplemental progesterone even if they take Clomid. If Clomid does not stimulate production and ovulation of a follicle, progesterone levels won't rise. Neither Clomid nor progesterone alone, or both together, will fix all the potential problems that can arise with embryo implantation.
If you take Clomid, your doctor will typically continue to have you monitor your progesterone levels after ovulation to make sure they rise appropriately. Progesterone supplements provide an easy fix for low progesterone levels. However, increasing progesterone levels will not prevent loss of an embryo with abnormal chromosomes after implantation. Clomid can also cause more than one follicle to develop; if you get pregnant with more than one embryo, your progesterone levels may rise higher than normal in early pregnancy.