Increasing amounts of circulating hormones bring about pregnancy changes throughout the body, and all body systems are affected to a greater or lesser degree. The changes allow the fetus to develop and grow, prepare the woman for labour and delivery, and prepare her body for lactation.
The reproductive system
• Most of the changes take place in the uterus, which undergoes hypertrophy and hyperplasia of the myometrium. The decidua also becomes thicker and more vascular.
• Progesterone causes the endocervical cells to secrete thick mucus, which forms a plug, called the operculum, in the cervical canal, protecting the pregnancy from ascending infection.
• Muscles in the vagina hypertrophy and become more elastic to allow distension during the second stage of labour.
The cardiovascular system
• Due to the increasing workload, the heart enlarges.
• Cardiac output increases to accommodate the increasing circulating blood volume.
• Peripheral resistance is lowered, due to the relaxing effect of progesterone on the smooth muscle of the blood vessels, leading to a fall in BP.
• To avoid aorto- caval compression, as the arterial walls are more relaxed, it is important to avoid placing the woman in an unattended supine position during the third trimester.
• Blood flow increases in the uterus, skin, breasts, and kidneys, and blood volume increases by 20– 50%, varying according to size, parity, and whether the pregnancy is singleton or multiple.
The respiratory system
• Oxygen consumption increases by 15– 20% at term.
• Tidal volume increases by 40%.
• Residual volume decreases by 20%.
• Alveolar ventilation increases by 5– 8L/ min, four times greater than oxygen consumption, resulting in enhanced gaseous exchange.
• The amount of air inspired over 1min increases by 26%, resulting in hyperventilation of pregnancy, causing carbon dioxide (CO2) to be removed from the lungs with greater efficiency.
• Oxygen transfer to, and CO2 transfer from, the fetus are facilitated by changes in the maternal blood pH and partial pressure of CO2 (pCO2).
The urinary system
• Renal blood flow increases by 70– 80% by the second trimester.
• The glomerular filtration rate (GfR) increases by 45% by 8 weeks’
gestation.
• Creatinine, urea, and uric acid clearance is increased.
• Glycosuria occurs as a result of the increased GfR and is not usually related to increased blood glucose.
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• The ureters relax under the influence of progesterone and become dilated. Compression of the ureters against the pelvic brim can lead to urinary stasis, bacteriuria, and infection of the urinary tract.
• As the fetal head engages at the end of pregnancy, the bladder may become displaced upwards.
The gastrointestinal system
• Nausea is experienced by 70% of pregnant women, beginning at around 4– 6 weeks and continuing until 12– 14 weeks.
• Most women notice an increased appetite and an increased thirst in pregnancy.
• Reflux of acid into the oesophagus, resulting in heartburn, is common.
• Transit of food through the intestines is much slower and there is increased absorption of water from the colon, leading to an increased tendency to constipation.
Skeletal changes
• Pelvic ligaments relax under the influence of relaxin and oestrogen, with the maximum effect in the last weeks of pregnancy.
• This allows the pelvis to increase its capacity to accommodate the presenting part during the latter stage of pregnancy and during labour.
• The symphysis pubis widens and the sacro- coccygeal joint loosens, allowing the coccyx to be displaced.
• whilst these changes facilitate vaginal delivery, they are likely to be the cause of backache and ligament pain.
Skin changes
• Increased pigmentation of the areola, abdominal midline, perineum, and axillae due to a rise in pituitary melanocyte- stimulating hormone (MSH).
• The ‘mask of pregnancy’, or chloasma, a deeper colouring of the face, develops in 50– 70% of women, is more common in dark- haired women, and is exacerbated by sun exposure.
• Striae gravidarum, commonly called stretch marks, occur as the collagen layer of the skin stretches over areas of fat deposition, e.g. breasts, abdomen, and thighs.
• The stretch marks appear as red stripes and change to silvery white lines within 6 months of delivery.
• Scalp, facial, and body hair become thicker. The excess is shed in the postnatal period.
The breasts
• Breast changes are one of the first signs of pregnancy noticed by the mother. from around 3– 4 weeks’ gestation, there is increased blood flow and tenderness; veins become more prominent, and the breasts feel warm.
• Under the influence of oestrogen, fat is deposited in the breasts, increasing their size. The lactiferous tubules and ducts enlarge.
• The pigmented area around the nipple darkens.
• Progesterone causes growth of the lobules and alveoli and develops the secretory ability of these structures, ready for lactation.
• Prolactin stimulates the production of colostrum from the second trimester onwards and, after delivery, is responsible for the initiation of milk production.
The endocrine system
All the endocrine organs are influenced by secretion of placental hormones during pregnancy.
• Pituitary hormones: prolactin, adrenocorticotropic hormone (ACTH), thyroid hormone, and MSH increase. follicle- stimulating hormone (fSH) and luteinizing hormone (LH) are inhibited. Oxytocin is released throughout pregnancy and increases at term, stimulating uterine contractions.
• Thyroid hormones: total thyroxine levels rise sharply from the second month of pregnancy. The basal metabolic rate is increased.
• Adrenal hormones: cortisol levels increase, leading to insulin resistance and a corresponding rise in blood glucose, particularly after meals. This makes more glucose available for the fetus.
• Pancreas: due to increasing insulin resistance, the β cells are stimulated to increase insulin production by up to four times during pregnancy.
In women with borderline pancreatic function, this may result in the development of gestational diabetes, affecting 3– 12% of pregnant women.
Further reading
McNabb M (2011). Maternal and fetal responses to pregnancy. In: Macdonald S, Magill- Cuerden J, eds. Mayes’ Midwifery: A Textbook for Midwives, 14th edn. edinburgh: Baillière Tindall, pp. 397– 410.
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