In early pregnancy, human chorionic gonadotrophin is secreted by the embryo and placenta, and stimulates the production of oestrogen and progesterone by the mother’s ovary until around the 10th week of pregnancy when the placenta takes over. These hormones are responsible for many of the physiological changes in the mother. The first change of note is a missed period because ovulation stops. There is a change in the normal rhythmic activity of the uterine muscle with longer resting phases, under the influence of progesterone, while the womb enlarges as a result of the increasing oestrogen secretion. If there is bleeding after a missed period advice should be sought from your doctor.
During pregnancy the placenta, fetus and sac secrete several steroid and protein hormones and growth factors, so that the mother’s body will adapt for pregnancy. The hormones produced include oestrogen, progesterone, human chorionic gonadotrophin (HCG), human chorionic somato-mammotrophin (HCS), and placental adrenocorticotrophic hormone (PAH). There are also specific growth factors secreted, such as the inhibins, and insulin-like growth factors which are produced by the placenta and the mother’s ovaries.
As a result of all these changes there is a big increase in the amount of blood circulating through the body. The womb enlarges to accommodate the growing fetus. The hormones change the levels of sugar and other blood contents, so as to make it easier for the placenta to take what it needs to feed the fetus. Some hormones relax the womb and other muscles, so that the fetus can squeeze into the pelvis for delivery. A more detailed outline of what happens in pregnancy is set out below.
Physiological changes in the mother – more technical information
In the mother there is an increase in the renal tubular reabsortion of salt (sodium), which plays an important role in regulating fluid volume. The excretion of sodium is regulated by many factors, which include the effect of hormones such as progesterone, aldosterone from the adrenal gland, and the natriuretic hormones e.g. Arginine-Vasopressin (AVP). The metabolic changes in pregnancy create a tendency towards a low potassium level (hypokalaemia), but this is balanced by the potassium sparing effect of progesterone. Serum uric acid levels are decreased because renal clearance is increased during pregnancy, and renal tubular reabsorption is reduced.
The maternal red blood cell mass normally increases by about 18% during pregnancy in response to increasing erythropoietin levels. This increase is not proportional to the much higher increase in plasma volume (41-57%) and therefore results in a state of relative haemodilution during pregnancy. The total white blood cell count is increased, especially among the neutrophils, and there is a slight reduction in the platelet count. There is an increase in the clotting factors, (VII, VIII, IX and X) and a reduction in fibrinolytic activity. This results in an increased tendency towards the formation of blood clots (thrombus) during pregnancy.
Glucose metabolism is altered because of a combined gradual reduction in insulin sensitivity as pregnancy progresses, and a reduced renal threshold, making glycosuria a common finding. The metabolic rate increases, in turn reflecting the increased demands placed upon maternal cardiac and pulmonary function. This is associated with increased thyroid activity, and mild thyroid gland enlargement is quite common in pregnancy. The total serum tri-iodothyronine and thyroxine concentrations are increased, following an a rise in serum binding globulin levels, but there is no increase in the free hormone concentrations. There is a significantly increased positive nitrogen balance during pregnancy, and there is an increased accumulation of lipid stores and calcium and iron.
Uteroplacental physiology – more detailed information
Certain structural changes occur in the uterine blood vessels during pregnancy, which are essential to the satisfactory formation of the placenta and development of an adequate fetoplacental circulation. The pivotal change occurs within the uterine arterial circulation especially the resistance vessels, the spiral arterioles. This is a continuous process throughout pregnancy. Invasion by non-villous trophoblast occurs in the first trimester, usually before the 12th week. Trophoblastic invasion of luminal smooth muscle has usually taken place by the 20th week. The process of placentation results in the conversion of the uterine circulation from a high resistance, low flow circuit, to a low resistance, high flow system, ensuring good communication between (dilated, tortuous spiral arterioles) uteroplacental arteries and the intervillous space. The pelvic venous vasculature also undergoes a process of maeked dilation, serving as a reservoir for the circuit in later pregnancy when large amounts of blood are circulated.
Fetal physiology – more detailed information
While the fetus eventually develops some degree of autonomy during pregnancy, structurally, it is the placenta that remains in direct contact with the external environment. In practise therefore, both fetus and placenta serve as a single functional unit for the purpose of vascular, respiratory, metabolic and endocrine activity.
The human fetus increases its weight 6 billion times from conception to delivery. This is by a combination of cellular multipication, and cellular growth (hyperplasia and hypertrophy respectively). Hyperplastic growth in mainly genetically determined while hypertrophy dependxs mainly on the environment. In the first 12 weeks, the fetoplacental system is not fully developed and pregnancy is supported mainly by tubal and endometrial secretion, under stimulation from the corpus luteum and other pregnancy hormones. Once the placental unit develops, growth is accelerated significantly and depends to a major extent on the normal function of this organ.
Further reading & information
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