A number of hormones are necessary for normal pregnancy to be maintained until parturition. These are secreted mainly by the placenta, and include:
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) 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:
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:
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:
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.
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:
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.
Guyton AC, Hall JE. Textbook of Medical Physiology. 11th ed. Elsevier Saunders.
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.
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