In order to meet with the demands of pregnancy, physiological adaptations occur in the mother. These adaptations allow her to support and protect the foetus.
In this article, we will take a systems-based approach to discuss the different changes which occur during pregnancy.
Endocrine System
During pregnancy, a woman experiences a change in her endocrine system. Throughout pregnancy the levels of progesterone and oestrogen increase; the oestrogen being produced by the placenta and the progesterone being produced by the corpus luteum and later by the placenta.
Increase in oestrogen levels results in an increase in hepatic production of thyroid-binding globulin (TBG). As a result, more free T3 and T4 bind to the TBG, this causes more thyroid-stimulating hormone to be released from the anterior pituitary gland. Therefore, the free T3 and T4 levels remain unchanged – but the total T3 and T4 levels rise.
Thyroxin is essential for the foetus’ neural development, but the foetal thyroid gland is not functional until the second trimester of gestation. Hence, increasing T3 and T4 levels in the mother ensures that there is a constant supply of thyroxin to the foetus early in pregnancy.
During pregnancy (mainly during the second trimester) there is also an increase in human placental lactogen, prolactin and cortisol levels. These are anti-insulin hormones therefore, they increase insulin resistance in the mother and reduce peripheral uptake of glucose. This ensures that there is a continuous supply of glucose for the foetus.
The mother switches to an alternative source of energy which is provided by lipids. The increase in lipolysis means that there is an increase in free fatty acids in the plasma which provide substrate for maternal metabolism. The breakdown of lipids can result in ketogenesis thus, pregnancy is associated with an increased risk of ketoacidosis.
Cardiovascular System
As discussed above, during pregnancy progesterone levels increases. Progesterone acts to decrease systemic vascular resistance in pregnancy which leads to a decrease in diastolic blood pressure during the first and second trimester of pregnancy.
In response to this the cardiac output increases by about 30-50%. An increase in blood pressure in pregnancy could be an indication of pre-eclampsia.
Pregnancy results in the activation of the renin-angiotensin-aldosterone system, leading to an increase in sodium levels and water retention. This means that the total blood volume increases.
Respiratory System
Anatomically, the growth of the foetus during pregnancy causes upward displacement of the diaphragm. However, this does not decrease the total lung capacity significantly as there is also an increase in the transverse and anterior-posterior diameters of the thorax.
In pregnancy, a woman faces an increase in their metabolic rate, which leads to an increased demand for oxygen. The tidal volume and the minute ventilation rate increases to help the mother meet these oxygen demands.
Many women experience hyperventilation during pregnancy, it is thought that the reason for this is the increased carbon dioxide production and the increased respiratory drive caused by progesterone. This hyperventilation results in a respiratory alkalosis with a compensated increase in renal bicarbonate excretion.
Gastrointestinal System
The growth of the uterus causes a number of anatomical changes related to the gastrointestinal tract. One of these is the upward displacement of the stomach as the uterus grows, this leads to an increase in intra-gastric pressure which would predispose the mother to gastrointestinal reflux, along with symptoms such as nausea and vomiting.
The appendix may also move to the right upper quadrant of the abdomen as the uterus enlarges.
The increase in progesterone during pregnancy results in smooth muscle relaxation, which decreases gut motility. Although this allows for more time for nutrient absorption, it can lead to constipation. Increased progesterone also causes relaxation of the gallbladder so biliary tract stasis may occur. This predisposes the mother to gallstones.
Urinary System
Increased cardiac output during pregnancy causes an increase in renal plasma flow, which increases the GFR by about 50-60%. This would mean that there is an increase in renal excretion, so in pregnancy, the levels of urea and creatinine will be lower.
Progesterone affects the urinary collecting system causing relaxation of the ureter (resulting in hydroureter) and relaxation of the muscles of the bladder. Both of these changes causes urinary stasis which predisposes a woman to UTIs, commonly pyelonephritis.
Figure 1 – Diagram showing the changes that occur in the urinary system during pregnancy
Haematological Changes
In pregnancy there is an increase in fibrinogen and clotting factors in the blood and a decrease in fibrinolysis. Additionally, due to an increase in progesterone levels stasis of blood and ventilation occurs.
All of these factors increase the risk of thromboembolic disease in pregnancy. Warfarin can not be given to pregnant women to counteract this as it can cross the placenta and it is a teratogen. Low Molecular Weight Heparin (LMWH) is usually considered the anticoagulant of choice during pregnancy if it is necessary to give the mother an anticoagulant drug.
During pregnancy, the plasma volume increases significantly, however, the red cell mass does not increase by as much. This results in physiological dilutional anaemia.
Clinical Relevance – Gestational Diabetes Mellitus (GDM)
Usually, as the pregnancy progresses there is an increase in insulin resistance however, normally this can be counteracted by increasing insulin production. In women with gestational diabetes, this compensatory increase in insulin levels does not occur which results in high blood sugar levels.
The diagnostic criteria for diagnosing a woman with gestational diabetes is as follows:
- Fasting plasma glucose level of 5.6 mmol/L or above;
- Two-hour plasma glucose level of 7.8 mmol/L or above.
Risk factors for developing GDM include age, high BMI before pregnancy, family history of type 2 diabetes and smoking.
Non-drug treatment for GDM includes changing diet and physical activity. Insulin can be given as treatment when lifestyle measures do not help to maintain blood sugar levels. Other agents such as metformin can also be offered.
GDM poses a risk to the mother and the baby. Two main risks to the baby are macrosomia in unmanaged GDM (which can lead to complications during birth as the baby is bigger in size) or intrauterine growth retardation in managed GDM. Although most women recover from GDM after pregnancy there is a chance that in some GDM will recur in future pregnancies.
Further information on gestational diabetes is available here.
