Physiological Changes in Pregnancy

Introduction[edit | edit source]

Pregnancy and the associated changes are a normal physiological process in response to the development of the fetus. These changes happen in response to many factors; hormonal changes, increase in the total blood volume, weight gain, and increase in foetus size as the pregnancy progresses. All these factors have a physiological impact on the pregnant woman: the musculoskeletal, endocrine, reproductive , cardiovascular, respiratory, nervous, urinary, gastrointestinal and immune systems are affected, along with changes to the skin and breasts. The full gestation period is 39-40 weeks, and a pre-term birth is classed as delivery before 37 weeks gestation, although there is variation internationally and it is thought that the length of human pregnancies also vary naturally.[1]

Anatomy Background[edit | edit source]

Pelvis anterior and posterior, segments highlighted.png
  • The pelvis is the region found between the trunk and lower limbs.
  • In females, the pelvis is wider and lower than that of their male counterpart, making it more suited to accommodate a foetus during both pregnancy and delivery[2].
  • It protects and supports the pelvic contents, provides muscle attachment and facilitates the transfer of weight from trunk to legs in standing, and to the ischial tuberosities in sitting.
  • The cross-sectional anatomy of the female pelvis shows five bones: two hip bones, sacrum, coccyx, and two femurs. The joints are supported by some of the strongest ligaments in the body which become laxer during pregnancy leading to increased joint mobility and less efficient load transfer through the pelvis.
  • The pelvic outlet at the base of the pelvis is narrower in its transverse diameter when compared with the pelvic inlet; it comprises the pubic arch, ischial spines, sacrotuberous ligaments, and coccyx.
  • Four pairs of abdominal muscles combine to form the anterior and lateral abdominal wall and may be termed the abdominal corset.
  • Transversus abdominis lies deep to the internal abdominal oblique and external abdominal oblique with the rectus abdominis central, anterior and superficial. The internal oblique, external oblique and transversus abdominis insert into an aponeurosis joining in the midline at the linea alba. The deep abdominal muscles, together with the pelvic floor muscles, multifidus, and diaphragm, can be considered as a complete unit and may be termed the lumbopelvic cylinder. This provides support for the abdominal contents and maintains intra­-abdominal pressure.
  • Organs of the female reproductive system present in the pelvis are subdivided into internal and external genitalia.
  • The internal genitalia consists of the uterus, two uterine tubes, two ovaries, and the vagina.
  • The external genitalia mainly consists of the mons pubis, clitoris, labia majors, labia minora, and Bartholin glands.

Endocrine System Changes[edit | edit source]

Pregnancy is associated with changes in hormone levels. These hormones work together to control the growth and development of the placenta and the foetus, and act on the mother to support the pregnancy and prepare for childbirth. Many organs of the body secrete hormones which affect the expectant mother, although when the placenta is developed it then takes over the production of many of these hormones, including: oestrogen, progesterone, human chorionic gonadotrophin (HCG), human placental lactogen, placental growth hormone, relaxin and kisspeptin.

HCG is the first hormone to be released from the developing placenta and is the hormone that is measured in a pregnancy test. It acts as a signal to the mother’s body that pregnancy has occurred by maintaining progesterone production.

Progesterone is initially produced by corpus luteum, a temporary endocrine gland found in the ovary. Progesterone maintains the pregnancy, by supporting the lining of the womb and preventing premature uterine contractions. It reduces the tone of smooth muscles (causing constipation due to the water retention in the colon), contributes to breast development, increases the storage of fat due to its catabolic effect on metabolism and increases body temperature.

Oestrogen, is also initially produced by the corpus luteum and later by the placenta. Oestrogen levels rise towards the end of pregnancy. Oestrogen acts to stimulate the growth of the uterus to accommodate the growing fetus, by having a vasodilation effect and increasing blood flow to the uterus. It allows the uterus to contract by countering the effect of progesterone and in this way, prepares the uterus for labour. Oestrogen also stimulates the growth and development of the breasts.

Relaxin causes the relaxation of pelvic ligaments and softening of the cervix at the end of pregnancy, which aids the process of labour.

Reproductive System Changes[edit | edit source]

During pregnancy, the internal genital tract undergoes anatomical and physiological changes to accommodate the changes and development of the foetus.

  • These changes are presented as below:
    Fundus levels during pregnancy

Uterus[edit | edit source]

  • With pregnancy progression, the uterus leaves the pelvis and ascends to the abdominal cavity
  • The abdominal content displaced in response to the increased size of the uterus which is five times more than normal
  • This increase in the size of uterus is associated with an increase of blood supply to the uterus and uterine muscle activity,
  • Uterus increases in size till the 38 weeks after that the fundus level starts to descend preparing for delivery.
  • Its weight increases from 50mg to 1000mg at 40 weeks and stretches to accommodate the foetus size, which is associated with an increase in the thickness and length of the fundus.

Cervix[edit | edit source]

The enlarged mucus glands of the cervix during pregnancy secrete a mucus, which forms a plug called the “operculum”. This acts as a seal for the uterus and protects it from ascending infection, and acts as a barrier between the vagina and cervix. Later in pregnancy before delivery, there is a softening of the cervix in response to oestrogen and progesterone. Ripening of the cervix occurs due to the effect of prostaglandin and relaxin as labour becomes imminent.

Vagina[edit | edit source]

The muscle layer of the vagina thickens and it becomes more elastic, making it possible for the vagina to dilate during the second phase of labour. The number of squamous cells increases, due to glycogen, which predisposes the vagina towards thrush. [3][4].

Musculoskeletal Changes[edit | edit source]

Swanger vrou2.jpg

Postural Changes[edit | edit source]

  • The overall equilibrium of the spine and pelvis alters as the pregnancy progresses[5]
  • There is still confusion as to the exact nature of any associated postural adaptation, with weight gain, increased blood volume, and ventral growth of the foetus,
  • The centre of gravity no longer falls over the feet, but instead shift posteriorly. There is an increase in anteroposterior and medial-lateral sway[6], and women may need to lean backwards to gain equilibrium resulting in disorganisation of spinal curves.
  • Reported postures include: a reduction in lumbar lordosis, an increase in both lumbar lordosis and thoracic kyphosis, or a flattening of the thoracolumbar spinal curve. 
  • There will be compensatory changes to posture in the thoracic and cervical spines, and this combined with the extra weight of the breasts may result in posterior displacement of the shoulders and thoracic spine, increase anterior pelvic tilting, and increase of the cervical lordosis.[7]
  • These changes may be still similar for 8 weeks after delivery.

Articular Changes[edit | edit source]

  • Altered levels of relaxin, oestrogen, and progesterone during pregnancy result in an alteration to collagen metabolism. This laxity is due to the break down of collagen in the targeted tissue which is replaced by a modified form that contains higher water content.
  • Connective tissue pliability and extensibility increases. Therefore, ligamentous tissues are predisposed to laxity with resultant reduced passive joint stability. Ligament laxity reaches its maximum at the second trimester[8].
  • The symphysis pubis and sacroiliac joints are particularly affected to allow for the birth of the baby. Ligamentous laxity may continue for six months postpartum. Bio-mechanical changes of the spinal and pelvic joints may involve: an increase in sacral promontory, an increase in lumbosacral angle, a forward rotatory movement of the innominate bones, and downward and forward rotation of the symphysis pubis.
  • The normal pubic symphyseal gap of 4–5 mm shows an average increase of 3 mm during pregnancy
  • Pelvic joint loosening begins around 10 weeks, with maximum loosening near term. Joints should return to normal at 4–12 weeks postpartum.
  • The sacrococcygeal joints also loosen. By the last trimester, the hip abductors, extensors, and the ankle plantar flexors increase their net power during gait and there is an increase in load on the hip joints of 2.8 times the normal value when standing and working in front of a worktop. As the uterus rises in the abdomen the rib cage is forced laterally and the diameter of the chest may increase by 10–15 cm.[7]

Neuromuscular Changes[edit | edit source]

  • During pregnancy, the enlarged uterus results in elongation of the abdominal muscles and separation of the linea alba.
  • Passive joint instability (as seen in pregnancy) alters afferent input from joint mechanoreceptors and probably affects motor neuron recruitment.
  • A decrease in muscle stiffness and thus active stability of joints may result from alteration of muscle spindle regulation and this is applicable particularly to muscles around the pelvic girdle.
  • These changes may lead to poor recruitment of the muscles responsible for pelvic girdle stability (particularly gluteus medius and Maximus) and result in decreased tension of these muscles during walking, perhaps resulting in pelvic girdle pain (PGP).[7]

Nervous System[edit | edit source]

  • Fluid retention can compress nerves passing through narrow canals, such as the carpal tunnel, causing pain, numbness and weakness in the hand.
  • Anxiety, increased mood lability, vivid nightmares and insomnia are well documented throughout preganancy, although the exact aetiology is unknown[9].

Cardiovascular changes[edit | edit source]

  • The heart adapts to the increased cardiac demand that occurs during pregnancy in many ways.
  • Cardiac output increases throughout early pregnancy, and peaks in the third trimester, usually to 30-50% above baseline.
  • Oestrogen mediates this rise in cardiac output by increasing the pre-load and stroke volume, mainly via a higher overall blood volume (which increases by 40–50%).
  • The heart rate increases, but generally not above 100 beats/ minute.
  • Total systematic vascular resistance decreases by 20% secondary to the vasodilatory effect of progesterone. Overall, the systolic and diastolic blood pressure drops 10–15 mm Hg in the first trimester and then returns to the baseline in the second half of pregnancy. 
  • All of these cardiovascular adaptations can lead to common complaints, such as palpitations, decreased exercise tolerance, and dizziness.[10]
  • Women may suffer from supine hypotension due to uterine compression of the vena cava.

Respiratory Changes[edit | edit source]

Pregnant breathing.jpg
  • Respiratory changes during pregnancy are important to accommodate and meet the demands of mother and foetus, there are changes in all lung volumes, changes in the upper airway respiratory tract, and breathing pattern.
  • There is increasing oedema in the upper airway tract, and if intubation was necessary a smaller endotracheal tube would be needed.
  • The diaphragm is elevated by about 4cm due to the enlarged uterus.
  • Ligaments connecting ribs to sternum become lax during pregnancy. The subcostal angle increases from 68 in early pregnancy to 103 in late pregnancy. Chest circumference increase from 5-7cm and this is associated with lower chest compliance.
  • Lung Volumes change as follows; functional residual capacity decreases by 10-25%, expiratory reserve volume  15-20%, residual volume is decreased by 20-25%, and the total lung capacity decreases. There is an increase in the respiratory capacity by 5-10%, respiratory rate by 1-2 breaths more than normal, and tidal volume by 30-50%.
  • We will find an increase in oxygen consumption by 30% and the metabolic rate by 15% in pregnant women, but they still have a lower oxygen reservoir due to the lower rate of functional residual capacity FRC. Pregnant women are more prone to hypoxia, hyperventilation and dyspnea than non-pregnant women.
  • In addition to these changes there is an increase in PaO2 to facilitate the transfer of oxygen from mother to foetus and lower PaCo2 to facilitate the transfers of carbon dioxide from foetus to mother[11][12].

Gastrointestinal changes[edit | edit source]

  • Progesterone causes smooth muscle relaxation which slows down GI motility and decreases lower oesophageal sphincter (LES) tone.
  • The resulting increase in intra-gastric pressure combined with a decrease in LES tone leads to the gastro-oesophageal reflux commonly experienced during pregnancy.
  • Nausea and vomiting of pregnancy, commonly known as “morning sickness”, is one of the most common GI symptoms of pregnancy. It begins between the 4 and 8 weeks of pregnancy and usually subsides by 14 to 16 weeks.
  • The exact cause of nausea is not fully understood but it correlates with the rise in the levels of human chorionic gonadotropin, progesterone, and the resulting relaxation of the smooth muscle of the stomach[13].
  • Constipation and haemorrhoids can occur during pregnancy, and are attributed to the smooth muscle relaxation, decreased motility of the bowel and increased water absorption of the colon.

Renal changes[edit | edit source]

  • A pregnant woman may experience an increase in the size of the kidneys and ureter due to the increased blood volume and vasculature.
  • Later in pregnancy, the woman might develop physiological hydronephrosis and hydroureteronephrosis, which are normal.
  • There is an increase in glomerular filtration rate associated with an increase in creatinine clearance, protein, albumin excretion, and urinary glucose excretion.
  • There is also an increase in sodium retention from the renal tube so oedema and water retention is a common sign in pregnant women[11].
  • In the third trimester when the foetus starts to engage in the pelvis, there is an increased frequency of urination. The uterus compresses the ureters at the pelvic brim, causing a slowing of urine flow which combined with an increase in urine output results in frequent trips to the toilet.
  • Stress and urge incontinence are common amongst pregnant women[9].

Nutrition[edit | edit source]

  • During pregnancy, both protein metabolism and carbohydrate metabolism are affected.
  • One kilogram of extra protein is deposited, with half going to the fetus and placenta, and another half going to uterine contractile proteins, breast glandular tissue, plasma protein, and haemoglobin.
  • Pregnant women require a caloric increase. There is an increased requirement for nutrients due to foetal growth and fat deposition.
  • Changes are caused by steroid hormones, lactogen, and cortisol.
  • A pregnant woman can expect to agin between 20 to 30 lb (9.1 to 13.6 kg)[14]depending on the pre-pregnancy weight. Weight gain or weight loss is a poor indication of foetal well-being.

Skin[edit | edit source]

  • Pigmentation changes occur during pregnancy including darkening of the areola on the breasts and the linea nigra, increased colouring on the vulva and increased facial pigmentation.
  • Stretch marks (striae gravidarum) occur on the abdomen, breasts, thighs and buttocks to varying degrees. They may occur due to changes in the elastic fibres and collagen in the dermis, which ruptures and overstretches the epidermis, causing the scarring.
  • Sayer et al (1990) found women with stress incontinence, prolapsed bladder neck had a greater incidence of abdominal striae.[9]
  • During pregnancy there is a marked reduction in normal hair loss, due to an increased growth phase of the hair follicles.

Breasts[edit | edit source]

  • Breast tenderness is common in the early stages of pregnancy due to enlargement under the influence of relaxin, progesterone and oestrogen. Breasts increase in weight by aproximately 500-800g.
  • Montgomery's tubercles developing form enlarging sebaceous glands around the areolar.

Immunity[edit | edit source]

  • Mother has some general depression of immunity so that she does not reject the foetus
  • Slightly increased risk of latent viruses reactivating e.g. influenza, pneumococcal pneumonia

Problems may have during pregnancy[edit | edit source]

  1. Pelvic floor dysfunction.
  2. Rib pain.
  3. Nerve compression syndromes.
  4. Carpal tunnel syndrome.
  5. Muscle cramps.
  6. Symphysis pubis dysfunction
  7. Morning sickness.
  8. Edema.
  9. Pre-eclampsia
  10. Back pain.

Exercise[edit | edit source]

Exercising whilst pregnant is essential to optimise the chance of a healthy, uncomplicated pregnancy. Learn more about physical activity during pregnancy.

See also

Pregnancy Related Pelvic Pain

Low Back Pain and Pregnancy

References[edit | edit source]

  1. Jukic A, Baird D, Weinberg C, McConnaughey D, Wilcox A. Length of human pregnancy and contributors to its natural variation. Human Reproduction, 2013
  2. Sapsford R. The pelvic floor and its related organs. In WB Saunders Women's Health: a textbook for Physiotherapists. 2004 pp56-72.
  3. Obstetric and Newborn Care 1. Changes of the Reproductive System during Pregnancy. [online] Accessed 21 Jul 2022.
  4. The Open University. Antenatal Care Module: 7. Physiological Changes During Pregnancy. Accessed 21 Jul 2022.
  5. Inanir A, Cakmak B, Hisim Y, Demirturk F. Evaluation of postural equilibrium and fall risk during pregnancy. Gait & Posture. 2014 Apr 1;39(4):1122-5. Accessed 21 Jul 2022
  6. Danna-Dos-Santos A, Magalhaes AT, Silva BA, Duarte BS, Barros GL, Maria De Fátima CS, Silva CS, Mohapatra S, Degani AM, Cardoso VS. Upright balance control strategies during pregnancy. Gait & posture. 2018 Oct 1;66:7-12. Accessed 21 Jul 2022.
  7. 7.0 7.1 7.2 Stuart Porter, Tidy's physiotherapy. 1991. Accessed 21 Jul 2022.
  8. Cherni Y, Desseauve D, Decatoire A, Veit-Rubinc N, Begon M, Pierre F, Fradet L. Evaluation of ligament laxity during pregnancy. Journal of gynecology obstetrics and human reproduction. 2019 May 1;48(5):351-7. Accessed 21 Jul 2022.
  9. 9.0 9.1 9.2 Sharpe R. Pregnancy and Puerperium: physiological changes, in w.b. Saunders Women's Health: a textbook for Physiotherapists. 2004 pp112-124.
  10. Sanghavi M, Rutherford JD. Cardiovascular physiology of pregnancy. Circulation. 2014 Sep 16;130(12):1003-8. Accessed 21 Jul 2022.
  11. 11.0 11.1 Tan EK, Tan EL. Alterations in physiology and anatomy during pregnancy. Best Practice & Research, Clinical Obstetrics & Gynaecology. 2013 Dec 1;27(6):791-802. Accessed 21 Jul 2022.
  12. Kohlhepp LM, Hollerich G, Vo L, Hofmann-Kiefer K, Rehm M, Louwen F, Zacharowski K, Weber CF. Physiological changes during pregnancy. Der Anaesthesist. 2018 May;67(5):383-96. Accessed 21 Jul 2022.
  13. Gomes CF, Sousa M, Lourenço I, Martins D, Torres J. Gastrointestinal diseases during pregnancy: what does the gastroenterologist need to know?. Annals of gastroenterology. 2018 Jul;31(4):385. Accessed 21 Jul 2022
  14. Forbes LE, Graham JE, Berglund C, Bell RC. Dietary change during pregnancy and women’s reasons for change. Nutrients. 2018 Aug;10(8):1032. Accessed 21 Jul 2022.