A number of hormones are necessary for normal pregnancy to be maintained until parturition. These are secreted mainly by the placenta, and include:
- Human chorionic gonadotropin (hCG)
- Human chorionic somatolactotropin (hCS)
- Steroid hormones, e.g. oestrogen and progesterone
- Growth hormone
- Corticotropin-releasing horone
- Gonadotropin-releasing hormone
These hormones are important in terms of maintaining a suitable environment and sufficient nutrition for the developing fetus, as well as during the processes of parturition and lactation. Of the aforementioned hormones, the placenta produces hCG, hCS, oestrogen and progesterone in much larger quantities. This article will focus on the effects of these 4 main hormones, as well as a brief overview on the hormonal factors leading up to parturition.
Human Chorionic Gonadotropin (hCG)
Human chorionic gonadotropin (hCG) is a glycoprotein with a heterodimeric structure closely related to that of luteinizing hormone (LH). Both hCG and LH (as well as FSH and TSH) share a common α-subunit, but hCG has a different β-subunit.
It is secreted by syncitial trophoblast cells derived from a fertilised ovum, and is first detectable 8-9 days after ovulation. hCG levels then increase rapidly in the initial 8-10 weeks of pregnancy to a peak of around 80,000-100,000 mIU/ml. This value decreases back to approximately 20,000 mIU/ml after about 16-20 weeks gestation and persists at this level throughout the rest of the pregnancy.
The function of hCG is similar to that of LH:
- Prevent degradation of the corpus luteum, therefore allowing the corpus luteum to maintain progesterone levels
- Progesterone is important for endometrial growth and prevention of menstruation, which is crucial for implantation of the blastocyst
- hCG maintains the decicual cells in the endometrial lining via sustained production of progesterone for about 3 to 4 months (after which progesterone production is taken over by the placenta)
- hCG also has a thyrotropic effect, which results in increased thyroid size and thyroxine production.
Human Chorionic Somatomammotropin (hCS)
hCS is a protein hormone produced by the placenta from approximately the 5th week of pregnancy. It is closely related to prolactin and GH, and increases in production with increased placental weight (steadily rising throughout pregnancy). hCS is secreted in particularly large amounts, with more hCS secreted by the placenta than all the pregnancy hormones combined.
Studies into the function of hCS have proved inconsequential, but several effects of hCS have been postulated:
- Believed to be involved in breast development and lactation. This action of hCS has been proven in animal models, but has not been shown to cause lactation in humans. Due to this function, hCS was initially known as human placental lactogen (HPL).
- Similar functions to growth hormone (GH). Both have similar structures, and both cause protein tissue formation via similar mechanisms. However, much more hCS is required to produce the same growth as GH.
- Anti-insulin effects and decreased insulin sensitivity in the mother. This avails larger quantities of glucose to the developing fetus, which uses glucose as its major substrate for growth.
- hCS increases levels of free fatty acids in the mother, providing an alternative source of energy during pregnancy. Due to this effect, hCS might be involved in the development of insulin resistance seen in pregnancy.
Oestrogen is secreted by the syncitial trophoblast cells, and is derived from androgen steroid precursors dehydroepiandrosterone and hydroxydehydroepiandrosterone. These are then converted to estradiol, estrone and estriol by the trophoblast cells.
Oestrogen is important during pregnancy because of its proliferative effects on tissues:
- Reponsible for enlargement of the uterus, breasts and breast ducts as well as the external genitalia of the mother
- Aids the ease of parturition via relaxation of the pelvic ligaments in the mother, which allows the sacroiliac joints and the pubic symphysis to become more limber.
- Do not cause contraction of uterus, but increase the capacity for contraction by the myometrial lining by increasing the number of prostaglandin/oxytocin receptors and gap junctions.
- Increases sensitivity of myometrium to both mechanical and chemical stimulation
As mentioned earlier, progesterone is produced initially by the corpus luteum before the placenta takes over after about 3-4 months. Once this occurs, progesterone levels increase rapidly throghout the remainder of pregnancy.
Progesterone is required if pregnancy is to be successful. This is related to its effects on both the mother and the developing embryo.
- Stimulates development of decidual cells in the endometrium
- Prevents spontaneous abortions by inhibiting any contractile action of the uterus.
- Increases secretions of the uterine tubes in the mother, which provide nutrition for the embryo
- May also have an effect on cell cleavage in the embryo.
- Works synergistically with oestrogen to prepare for lactation by causing proliferation of the breast lobules.
Hormones and Parturition
Parturition refers to the process of birth of the baby. This involves intense, rhythmic contractions of the uterus resulting in expulsion of the baby. There are both hormonal and mechanical changes that cause this. This article focuses on the hormonal factors to lead up to parturition.
The proportion of oestrogen to progesterone is an important factor, as progesterone inhibits contractions of the uterus while oestrogens increase contractions. Levels of both hormones increase throughout pregnancy, but progesterone levels taper off or even decrease from the 7th month onwards while oestrogen continues to rise. The increased ratio of oestrogen then leads to increased uterine contractility.
Oxytocin is secreted by the posterior pituitary (neurohypophysis). It is believed to have a specific action of increasing both the intensity and frequency of uterine contractions for 4 main reasons:
- In the final stages of pregnancy, there is an increased responsiveness of uterine myometrium to oxytocin due to an increased number of oxytocin receptors
- The rate of oxytocin production increases sharply during labour (stimulated by high oestrogen levels as well as mechanical distortion of the cervix)
- Labour is prolonged in animals who have had their neurohypophysis removed
- Stretching of the uterine cervix in animal models causes a reflex that increases oxytocin secretion
The fetus controls the timing of labour by increasing the supply of androgen precursors for estriol production, via activation of the fetal HPA axis. These precursors are calleddehydroepiandrostenedione sulphate (DHEAS). This contributes to a hyperoestrogenic state, together with placental estrone and estradiol which are derived from maternal androgens.
Cortisol is also produced by fetal adrenal glands. This helps to prepare the fetus to adapt to extrauterine conditions, and also increases expression of CRH, oxytocin and prostaglandins by the placenta.
Corticotropin releasing hormone (CRH) stimulates more DHEAS production via the fetal HPA axis, thus initating a positive feedback loop.
Hormonal Regulation of Parturition
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Norwitz E, Schorge J. Obstetrics and Gynecology at a Glance. 3rd Ed. Wiley-Blackwell.
Martini FH, Nath JL. Fundamentals of Anatomy and Physiology. 8th Ed. Pearson/Benjamin Cummings.