How the body changes in pregnancy

Pregnancy and associated changes are a normal physiological process of the woman’s body in response to the development – first – of the embryo – and then – of the fetus.

From the moment the embryo is implanted, the muscle fibers of the uterus thicken, the placenta and amniotic fluid begin to form, the blood volume increases and the breast prepares to increase its fat deposits and develop milk ducts, etc.

In another section of the site we talked about these changes following a timeline, or a time schedule – that of the weeks of pregnancy – in this article we see instead one by one the main maternal systems involved.

Endocrine system changes

Pregnancy is associated with changes in hormone levels. These hormones control the growth and development of the placenta and fetus and act on the mother’s body to support pregnancy and prepare for childbirth. Some of these hormones are: estrogen, progesterone, human chorionic gonadotropin (HCG), human placental lactogen (HPL), relaxinCRH hormoneoxytocin.

Beta HCG is the first hormone to be released from the developing placenta and is the hormone that is measured with pregnancy tests. Beta HCG induces the maintenance of the corpus luteum resulting in the production of progesterone.

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

Estrogens are also initially produced by the corpus luteum and later by the placenta. Estrogen acts to stimulate the growth of the uterus to accommodate the growing fetus, having a vasodilating effect and increasing blood flow to the uterus. It allows the uterus to contract by counteracting the effect of progesterone and, in this way, prepares the uterus for labor. Estrogen also stimulates breast growth and development. Estrogen levels increase towards the end of pregnancy.

Finally, relaxin causes relaxation of the pelvic ligaments and softening of the cervix at the end of pregnancy, which helps the process of labor.

Changes in the reproductive system

During pregnancy, the internal female genital organs undergo anatomical and physiological changes to accommodate the changes and development of the fetus.


With the progression of pregnancy, the uterus from the pelvis rises into the abdominal cavity. This displacement – which occurs in response to the increase in its size – causes the abdominal contents to displace. Increased uterine size is associated with increased blood supply to the uterus and uterine muscle activity.

The increase in the size of the uterus, which reaches a size five times larger than the initial ones, occurs until the 38th week of pregnancy, after which the level of the fundus begins to lower to prepare for childbirth.

From the first to the 40th week of pregnancy the weight of the uterus increases from 50 mg to 1000 mg.


Starting from conception, the mucous glands of the cervix secrete a mucus, which forms a plug called the mucous plug , whose function is to keep the cervical canal “sealed”, protect it from ascending infection and act as a barrier between the vagina and the cervix.

In later stages of pregnancy, before delivery, there is a softening of the cervix in response to estrogen and progesterone.

Maturation of the cervix occurs due to the effect of prostaglandins and relaxin as labor becomes imminent.

To learn more about the cervix in pregnancy, refer to our in-depth analysis.


During the second phase of labor the muscular layer of the vagina thickens and becomes more elastic, allowing the vagina to dilate.

Musculoskeletal changes

Postural changes

As pregnancy progresses, the mother’s posture and gait are altered; The center of gravity shifts forward, increasing lumbar lordosis, while the head and trunk are carried further back. This change has an impact on gait, as the body swings sideways and the legs are separated slightly more than normal, producing movement in the frontal plane. These variations reduce the propulsive force.

Increased sagittal pelvic inclination can result in hyperextension of the knees, weakening the extensor and flexor muscles of the knee and the extensors and abductors of the hip, and the shortening of certain muscle groups, such as the external adductors and rotators of the hip and the plantar flexors of the hip ankle. As a result, the sacrum acquires a more horizontal position and the intervertebral space decreases.

Joint changes

Hormonal changes (relaxin, estrogen and progesterone) determine an alteration of collagen metabolism, an important component of bones, cartilage, tendons, ligaments (but also teeth, skin and blood vessels) that in the bones confers resistance to stretching and in the cartilage allows the joints to resist tensile forces.

Biomechanical changes in the spinal and pelvic joints can result in an increase in the so-called sacral promontory, an increase in the lumbosacral angle, and a downward and forward rotation of the pubic symphysis. Also in this case these changes occur mainly to favor the passage of the fetus during childbirth.

Loosening of the pelvic joint begins around the tenth week of pregnancy to return to normal at 4-12 weeks after delivery.

The sacrococcygeal joints also loosen. By the , hip abductors, extensors and plantar ankle flexors increase their net power during gait, and there is an increase in the load on the hip joints by 2.8 times the normal value.

As the uterus lifts into the abdomen, the rib cage is forced laterally and the diameter of the chest may increase by 10-15cm.

Neuromuscular changes

During pregnancy the enlargement of the uterus causes the abdominal muscles to stretch and the Alba Line to separate .

Passive joint instability alters the afferent input of articular mechanoreceptors and probably affects motor neuron recruitment.

A decrease in muscle stiffness and therefore in the active stability of the joints can result from the alteration of the regulation of the muscle spindle and this is applicable in particular to the muscles around the pelvic girdle.

These changes can lead to poor recruitment of the muscles responsible for pelvic girdle stability (particularly the gluteus medius and gluteus maximus) and result in decreased tension in these muscles during walking, possibly resulting in pelvic girdle pain.

Nervous system

Fluid retention can compress nerves that pass through narrow channels, such as the carpal tunnel, causing pain, numbness, and weakness in the hand.

Anxiety, increased mood lability, vivid nightmares, and insomnia are common during pregnancy, although the exact etiology is unknown.

Cardiovascular alterations

The heart adapts to the increased cardiac demand that occurs during pregnancy in several ways:

  • estrogens mediate this increase in cardiac output by increasing preload and stroke volume, primarily through higher overall blood volume (increasing by 40-50%);
  • the heart rate increases, generally without exceeding 100 beats / minute;
  • Total systematic vascular resistance decreases by 20% depending on the vasodilating effect of progesterone. Overall, in the first trimester and then returns to baseline in the second half of pregnancy;

All of these cardiovascular adaptations can lead to common ailments, such as palpitations, decreased exercise tolerance, and dizziness.

Future mothers can also suffer from vena cava syndrome, due to uterine compression of the vena cava.

Respiratory changes

During pregnancy, changes occur in all lung volumes, changes in the respiratory tract of the upper airways and respiratory pattern. The goal of these changes is obviously to accommodate and meet the changing needs of the mother and those of the fetus.

At the 37th week of pregnancy the diaphragm is raised by about 4 cm due to the enlarged uterus.

During pregnancy the ligaments that connect the ribs to the sternum relax.

The subcostal angle increases from 68.5° – at the beginning of pregnancy – to 103.5° at the end of pregnancy. The girth of the chest increases by 5-7 cm.

Lung volumes change as follows:

  • residual functional capacity (volume present in the lungs at the end of normal breathing) decreases by 10-25%;
  • the expiratory reserve volume (maximum exhalable volume from the end of normal inspiration) decreases by 15-20%;
  • the residual volume (air that remains in the lung at the end of a maximum exhalation) decreases by 20-25%;
  • the total lung capacity (maximum amount of air contained in the lung) decreases;
  • there is an increase in inspiratory capacity (maximum volume inspired starting from the volume of end of normal exhalation) of 5-10%;


  • respiratory rate increases by 1-2 breaths more than normal;
  • The tidal volume (amount of air that is mobilized in a breathing act) increases by 30-50%.

These changes are accompanied by an increase in oxygen consumption by 30% and metabolic rate by 15%. In addition, 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 (arterial partial pressure of O2 in the blood or the state of oxygenation of the blood) to facilitate the transfer of oxygen from mother to fetus and a reduction in PaCo2 (partial pressure of carbon dioxide) to facilitate the transfer of carbon dioxide from the fetus to the mother.

Gastrointestinal changes

Progesterone causes smooth muscle relaxation which slows down gastrointestinal motility and decreases the tone of the lower esophageal sphincter.

Increased intragastric pressure combined with a decrease in the tone of the lower esophageal sphincter leads to gastroesophageal reflux commonly experienced during pregnancy.

Constipation and hemorrhoids can occur during pregnancy and are attributed to smooth muscle relaxation, reduced intestinal motility and increased water absorption by the colon.

Renal alterations

A pregnant woman may experience an increase in the size of the kidneys and ureter due to increased blood volume and vascularity.

In the late stages of pregnancy, the woman may develop hydronephrosis and hydroureteronephrosis (in hydronephrosis, urine accumulates only in the pelvis which, as a result, becomes larger than normal; in hydroureteronephrosis dilation extends to the ureter). There is an increase in glomerular filtration rate associated with increased clearance of creatinine, protein, albumin excretion and urinary glucose excretion. There is also an increase in sodium retention from the kidney tube, so water retention is a common disorder in pregnant women.

Also in the third trimester the uterus compresses the ureters at the pelvic edge, causing a slowdown in urine flow which combined with increased urine production results in frequent trips to the bathroom. Also in the is another common disorder among pregnant women.

Nutrition, caloric needs and body weight

As we have seen above during pregnancy the woman’s body undergoes many changes, both physical and hormonal. This means that the need for fats, carbohydrates and especially proteins increases: depending on the weight and the trimester, the woman will have to eat 70 to 100 g of protein per day. Good sources of protein are: lean beef and pork, chicken, salmon, peanuts, peanut butter, cottage cheese, beans.

In terms of calories, the additional requirement is 350 kcal per day for the second trimester and 460 kcal per day for the third trimester.

Some weight gain – from 9 to 14 kg – is normal during pregnancy: the extra body fat partly serves to provide nourishment to the fetus and partly is stored for breastfeeding after the birth of the baby.


During pregnancy, pigmentation changes occur, including darkening of the areola on the breast and linea nigra, increased staining of the vulva and increased pigmentation of the face.

Stretch marks on the abdomen, breasts, thighs and buttocks occur due to changes in the collagen fibers in the dermis, which break and stretch the epidermis excessively, causing scarring.

During pregnancy there is a marked reduction in normal hair loss, due to a greater growth phase of hair follicles.


One of the first signs for a pregnant woman is the change in the breasts, due to the progesterone produced during pregnancy. The breasts become more tense and the nipples are more sensitive as milk-producing lobules grow. The breasts and areolas begin to become larger, the nipples and areola become darker and remain so throughout pregnancy. The breasts gain weight by about 500-800 g.

The developing Montgomery’s tubercles form enlarged sebaceous glands around the areolar.

Immune system changes

During pregnancy the immune system undergoes a remodulation, which varies according to the gestational age.

The changes in the immune system thatoccur during pregnancy can be divided into two phases:

  1. in the 1st trimester the pro-inflammatory response prevails, to ensure implantation and totolerate the partial allogenicity of the fetus;
  2. In the 2nd and 3rd trimester, the anti-inflammatory response prevails, in preparation for childbirth.

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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