Estrogen and progestogens are sex hormones produced by the ovaries in women during the menstrual cycle and during pregnancy. Both of these hormones are found in the highest concentrations during pregnancy and act as a signal to the body not to produce and release an egg from the ovaries. For this reason, progesterone and estrogen are used together in the combined contraceptive pill to prevent unwanted pregnancy.
Sex Hormones
Progestogens form a class of steroid hormones that are produced in the body, and progesterone is the main hormone from this class that is produced naturally. For this reason, the terms are often used interchangeably. Estrogen and progestogens are both female sex hormones produced from a cholesterol precursor. The reaction pathway that synthesizes these hormones belongs to a group of synthesis pathways which all start with cholesterol and produce a variety of other important hormones, including testosterone. Both estrogen and progesterone are produced in the ovaries of the female at different stages of the menstrual cycle and during pregnancy.
Estrogen
The precursor to estrogen is the male sex hormone testosterone, which is produced in small amounts in women, and its role is to initiate sexual development in girls during puberty and to maintain fertility later on. It transforms body shape by causing changes in fat deposition and by causing breast growth, and it also plays an important role in the production of bone. During the menstrual cycle, it is released in greatest quantities during the pre-ovulatory phase when it prepares the uterus lining for implantation by a conceptus. During pregnancy, it is released from the placenta together with progesterone to maintain pregnancy. Without it, pregnancy would not be sustainable.
Progestogens
The progestogen progesterone is also produced during the menstrual cycle but it presents in highest quantities in the luteal phase after ovulation has occurred. Its role is to assist estrogen in maintaining the uterus lining and preventing it from being shed, as would normally occur when you have a period. During pregnancy, it is also produced from the placenta and levels can reach 40 times their post-ovulatory levels; however, progesterone levels drop sharply when birth occurs.
Progesterone and Estrogen During Pregnancy
Progestogens and estrogen are released in the highest quantities when you are pregnant and they act as a signal to the body to stop ovulation. They are therefore commonly used together as a contraceptive to convince the ovaries not to release any more eggs for fertilization. This contraception is known as the combined oral contraceptive pill, or COCP, and is taken daily at the same time for three consecutive weeks followed by a week of inactive or no pills when you have a period.
Mechanism of Action
The mechanism of action involves a complex series of sex hormones released from the brain and the ovaries which reciprocally interact, causing the various stages of the menstrual cycle. A part of your brain called the pituitary gland releases follicle-stimulating hormone and luteinizing hormone, which trigger the formation of an egg in your ovaries and cause the release of the egg during ovulation. High levels of progestogens and estrogen feed back on your pituitary gland in a negative feedback loop and lower the levels of FSH and LH, thereby inhibiting the release of an egg.
Safety of the Combined Pill
As of 2010, there is much debate about the long-term safety of the combined oral contraceptive pill. The Family Planning Association carried out a review in 2005 which looked at the studies investigating the link between breast cancer and the combined pill and found very conflicting data. Some studies showed no links at all and some showed an increased incidence of breast cancer in women taking the combined pill. There have also been suggestions of links with thromboembolisms, although again this is controversial. For more information on the combined pill, consult your physician.
References
- RCN: Hormones of the Reproductive System
- Encyclopedia Britannica: Synthesis of and secretion of estrogen
- Science Daily: Estrogen Maintains Pregnancy, Triggers Fetal Maturation
- Contraceptive Efficacy; Trussel; 2007
- "The reproductive system"; Rang and Dale's Pharmacology; R. Flower, H. Rang, M. Dale, J.Ritter; 2007
