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A full-term pregnancy encompasses approximately 270 days, equivalent to roughly 38.5 weeks, from the moment of conception to the birth of the infant. For practical purposes, healthcare providers often rely on the woman's last menstrual period (LMP) date to establish the estimated due date. This convention is based on a standard assumption that conception typically occurs on the 14th day of the woman's menstrual cycle. As a result, the due date is typically calculated as 284 days, or approximately 40.5 weeks, from the LMP.
This 40-week timeframe is customarily divided into three trimesters, each spanning approximately 13 weeks. Notably, during the second and third trimesters, the pre-pregnancy uterus, which initially approximates the size of a human fist, undergoes substantial growth to accommodate the developing fetus. This growth results in several significant anatomical changes in the expectant mother, which are essential to support and nurture the growing life within her (Figure 14).
Figure 14 | Changes in uterine dimensions during gestation | 23 This diagram visually represents the remarkable growth of the uterus from its pre-pregnancy size to its expansion by the end of the first, second, and third trimesters, adapting to the developing fetus.
Weight Gain
Profound alterations in maternal anatomy and physiology characterize the second and third trimesters of pregnancy. A prominent anatomical manifestation of pregnancy is the significant enlargement of the abdominal region, accompanied by maternal weight gain. This weight gain is attributable to multiple factors, including the growing fetus, an enlarging uterus, increased amniotic fluid volume, and the expanding placenta. Furthermore, augmented breast tissue and a substantial rise in blood volume also contribute to this overall weight gain (Table 1). Surprisingly, fat storage represents only a portion of this weight gain, accounting for approximately 2.3 kg (5 lbs) in a typical pregnancy. This fat reserve serves as an energy reservoir to meet the heightened metabolic demands of breastfeeding.
During the first trimester, there is no imperative need for the mother to increase her calorie intake to sustain a healthy pregnancy. However, it is common for women to experience a weight gain of approximately 0.45 kg (1 lb) each month during this period. Maternal appetite typically increases as pregnancy progresses into the second and third trimesters. Yet, to effectively support the developing fetus, only an additional 300 calories per day are required. Most women tend to gain around 0.45 kg (1 lb) per week during the later stages of pregnancy. This carefully regulated metabolic adjustment is essential for maternal and fetal well-being, ensuring the healthy growth and development of the unborn child without excessively burdening the mother.
|
Component |
Weight (lbs) |
|---|---|
|
Fetus |
7-8 |
|
Placenta and fetal membranes |
2-4 |
|
Amniotic Fluid |
2-3 |
|
Breast Tissue |
2-3 |
|
Blood |
4 |
|
Fat |
3-9 |
|
Uterus |
2-5 |
|
Total |
22-36 |
Changes in Organ Systems During Pregnancy
As the woman’s body adapts to pregnancy, characteristic physiologic changes occur throughout the body. In the uterus, smooth muscle contraction and relaxation are tightly regulated, and the balance between cAMP and cGMP signaling pathways plays a crucial role. In early pregnancy, cGMP may contribute to maintaining uterine quiescence (relaxation). Increased levels of cGMP have been associated with reduced myometrial contractility, helping to prevent premature contractions and maintaining a suitable environment for the developing fetus. As a woman approaches term and labor, there is a shift in the balance between cAMP and cGMP. Increased cGMP levels have been reported to enhance the effects of other signaling pathways that promote uterine contractions, contributing to the initiation and progression of labor. After childbirth, cGMP may play a role in the resolution of uterine involution.
Endocrine Changes
Many physiological changes that occur during pregnancy can predominantly be attributed to the intricate interplay of hormones. Key hormonal players in pregnancy, particularly estrogens, progesterone, and hCG, orchestrate a series of critical processes. In the initial stages, estrogen surges, originating from sources like the corpus luteum and, later, the placenta, fostering fetal growth and preparing the uterine lining. As pregnancy progresses, estrogen peaks in the second trimester. In the late third trimester, estrogen, while still elevated, experiences a modest decline. Simultaneously, progesterone rises substantially from the corpus luteum to maintain the uterine lining and prevent premature contractions by decreasing prostaglandin production, and lowering the uterus's sensitivity to oxytocin. Estrogen is initially produced by the corpus luteum during the first 5-6 weeks of pregnancy under the stimulation of hCG, after which the placenta takes over its production using DHEA from the fetus. It plays a vital role in promoting the continuous growth of the uterine myometrium and increased uterine blood flow. Estrogen, in conjunction with progesterone, supports the growth of ductal tissue in the breast and, along with relaxin, helps relax and soften maternal pelvic tissues to accommodate the expanding uterus. Near term, the estrogen-progesterone ratio increases, preparing the body for labor. Estriol, a form of estrogen excreted in the urine, serves as an important indicator of fetal well-being.
Progesterone is crucial for the establishment and sustenance of the morula and blastocyst, as it maintains the decidual lining of the uterus, providing essential nutrition for the developing fetus. Produced primarily by the placenta, with 90% of it entering maternal circulation, progesterone serves as a major substrate for cortisol and aldosterone synthesis in the fetal adrenal gland.
hCG, detected by pregnancy tests, sees rapid elevation early on, peaking by week 9 of pregnancy at which point the corpus luteum begins to regress, and the placenta assumes its endocrine functions (Figure 15). It is secreted by cytotrophoblasts and can be detected in maternal blood and urine within 8 days of conception. Its primary function is to maintain the corpus luteum beyond its normal lifespan, leading to increased production of progesterone and estrogen by the corpus luteum. Although hCG levels gradually diminish after this peak, they remain detectable throughout pregnancy. It enhances the production of dihydroepiandrostenedione (DHEA) in the fetal adrenal gland, which is used by the placenta to synthesize estrogens, and stimulates testosterone production in the male fetus. hCG receptors are found in the endometrium and myometrium, where they help reduce uterine contractions induced by oxytocin. It also acts as an immunosuppressant to protect the developing fetus.
Figure 15 | Estrogen, Progesterone, and Human Chorionic Gonadotropin Levels during Pregnancy | 24 This graph shows the relative concentrations of human chorionic gonadotropin (hCG), progesterone, and estrogens (total) in maternal circulation from conception to full term (40 weeks of pregnancy). Note the peak of hCG in the first trimester, followed by increasing levels of progesterone and estrogens throughout gestation with a decline in progesterone levels during the very end of pregnancy.
Cortisol contributes to the maturation of fetal organs like the lungs, liver, and various endocrine glands, such as the thyroid and adrenal glands. Simultaneously, they stimulate growth in maternal tissues, resulting in the enlargement of the uterus and the expansion and branching of mammary ducts.
Another hormone, relaxin, secreted by the corpus luteum and later by the placenta, plays a vital role in preparing the mother's body for childbirth. It enhances the flexibility of the symphysis pubis joint and pelvic ligaments, allowing the pelvis to accommodate the growing fetus and expand the pelvic outlet for delivery. Relaxin also facilitates cervical dilation during labor.
Furthermore, the anterior pituitary gland enlarges during pregnancy and escalates its hormone production, which leads to elevated levels of TSH, prolactin, and ACTH. TSH, in conjunction with placental hormones, increases the production of thyroid hormones, subsequently elevating the maternal metabolic rate, which can significantly augment the pregnant woman's appetite and give rise to hot flashes. Prolactin stimulates the enlargement of mammary glands in preparation for milk production. ACTH, on the other hand, stimulates maternal cortisol secretion, thereby contributing to fetal protein synthesis. In addition to the pituitary hormones, heightened parathyroid hormone levels trigger the release of calcium from maternal bones, facilitating its utilization by the fetus.
Estrogens further stimulate the production of angiotensinogen in the liver and encourage renin production by the kidneys, resulting in the heightened synthesis of angiotensin II and aldosterone. Although aldosterone secretion increases during pregnancy, clinical manifestations of hyperaldosteronism are typically not observed due to the counteracting influence of progesterone on aldosterone's renal effects.
Hematologic Changes
Pregnancy represents a unique physiological state characterized by chronic volume overload, leading to profound changes in fluid balance. Total body water, which typically starts at around 6.5 liters, expands to 8.5 liters by the end of pregnancy. At delivery, the fetus, placenta, and amniotic fluid collectively contribute approximately 3.5 liters of water.
One of the most notable physiological adaptations during pregnancy is the significant increase in blood volume. By the time of delivery, blood volume surpasses pre-pregnancy levels by approximately 30%, equivalent to an additional 1 to 2 liters. Plasma volume increases by about 25–40%, while red blood cell volume rises by 18 percent. However, this relative increase in plasma can cause the hematocrit to appear lower, a phenomenon often referred to as physiologic anemia of pregnancy. Additionally, leukocyte production is elevated during this time. The expanded blood volume is essential for meeting the heightened nutritional demands of the fetus and ensuring the efficient removal of fetal waste products.
Notably, water retention surpasses sodium retention in pregnancy, which leads to a remarkable 40 to 50 percent increase in plasma volume, accompanied by a decrease in osmolality. As a result, the total plasma protein concentration decreases due to the dilutional impact of the augmented intravascular fluid volume. Simultaneously, RBC volume also experiences an 18 percent increase. However, considering the rise in plasma levels, the concentration of RBCs per mm³ decreases from 4.5 million to 3.8 million, leading to a reduction in hematocrit levels. This phenomenon contributes to the relative anemia observed during pregnancy. Additionally, the number of leukocytes in the blood increases. Other associated changes include heightened platelet turnover, elevated fibrinogen levels, and an increased tendency for clot formation (hypercoagulability). These changes are part of the body's preparation for labor and delivery and to reduce the risk of excessive bleeding during delivery. There is also an increase in the activity of most clotting factors and enhanced fibrin degradation. Decreased activity of Protein S and increased resistance to Protein C create a pro-thrombotic environment, contributing to consumption coagulopathy, also known as disseminated intravascular coagulation (DIC), where excessive clot formation and fibrin degradation strain the coagulation system. Consequently, there is an elevation in the levels of D-dimers and fibrin degradation products. These changes in the coagulation profile during pregnancy contribute to an increased risk of deep vein thrombosis (DVT), emphasizing the need for careful monitoring and management of coagulation parameters in pregnant individuals.
The expansion in intravascular fluid volume serves as a critical physiological adaptation to compensate for the expected 300 to 500 mL blood loss associated with vaginal delivery and the average 800-1000 mL blood loss accompanying cesarean section.
Cardiovascular Changes
Pregnancy triggers remarkable cardiovascular adaptations to accommodate the increased physiological demands of the mother and fetus. Cardiac output rises substantially, beginning with a 35% increase by 12 weeks of gestation and stabilizing at 40–50 percent above pre-pregnancy levels for the remainder of pregnancy. Approximately 10 percent of this increased output is directed to the gravid uterus by term. The rise in cardiac output is achieved through two primary mechanisms: a 25–30 percent increase in stroke volume and a 15–25 percent elevation in heart rate. These changes are accompanied by a concurrent decrease in systemic vascular resistance, driven by the placenta's function as an arteriovenous shunt, leading to a reduction in diastolic pressure. Additionally, about 50 percent of pregnant individuals experience reversible cardiac hypertrophy, which may result in conduction changes. Auscultation often reveals a third heart sound (physiological S3) and a split S1 caused by the mitral valve closing slightly earlier than the tricuspid valve.
Labor initiates further increases in cardiac output, with the most significant surge occurring immediately postpartum due to autotransfusion from uterine contraction and the release of aortocaval compression. Postpartum cardiac output may increase by up to 80 percent, posing risks for individuals with preexisting cardiac conditions. Despite these changes, systemic vascular resistance remains low throughout pregnancy, and blood pressure typically does not rise until late gestation. However, aortocaval compression by the growing uterus can impair venous return from the lower body, leading to hypotension and symptoms such as nausea, diaphoresis, and dizziness, collectively known as supine hypotension syndrome. Compensatory mechanisms, such as increased venous pressure below the site of compression and diversion of blood through the paravertebral venous plexuses and azygos vein, help maintain venous return and systemic blood pressure. Reflex increases in sympathetic nervous system activity also contribute to preserving vascular resistance and blood pressure.
Aortocaval compression can significantly affect uterine and placental blood flow. Prolonged maternal hypotension exceeding 10–15 minutes may reduce uterine blood flow, risking progressive fetal acidosis. Additionally, venous compression increases the risk of venous puncture during epidural or spinal anesthesia due to blood redistribution through the internal vertebral venous plexus. To minimize complications, supine positioning should be avoided during anesthetic administration in the second and third trimesters. Anesthetic techniques that interfere with sympathetic nervous system activity may further compromise compensatory mechanisms, heightening the risk of profound hypotension and compromising maternal and fetal well-being.
Respiratory Changes
During pregnancy, the pulmonary system undergoes a series of physiological adaptations to support the growing fetus and meet increased oxygen demands. Initially, changes occur in the upper respiratory tract, including capillary engorgement in the mucosal lining, often resulting in nasal congestion and nosebleeds, particularly in cold or dry weather. Edema in the vocal cords and arytenoids may also develop, necessitating caution during airway instrumentation. As pregnancy progresses, the growing fetus and uterus displace the diaphragm upward, leading to significant shifts in lung volume dynamics. Functional residual capacity (FRC) and residual volume decrease, while vital capacity, FEV1, and the FEV1-to-forced vital capacity ratio remain unchanged. By the end of pregnancy, FRC is reduced to 80 percent of the non-pregnant value, and in the supine position, it decreases further to 70 percent of its sitting value. Additional factors such as regional anesthesia and obesity can further exacerbate this reduction, creating a compounding effect on diminished FRC.
To compensate for these changes, tidal volume increases by 40 percent, resulting in a 50 percent rise in minute ventilation and enhanced oxygen intake. These adjustments ensure sufficient arterial oxygenation for both the mother and fetus. Expiration and carbon dioxide output also increase, leading to lower maternal blood carbon dioxide levels and mild respiratory alkalosis, which facilitates gas exchange with the placenta. Progesterone plays a critical role in this process by directly influencing the CNS, lowering the set-point for carbon dioxide regulation, and promoting hyperventilation. Maternal carbon dioxide tension (PCO2) decreases from 40 mm Hg to approximately 32 mm Hg, resulting in compensated respiratory alkalosis characterized by reduced serum PCO2 and bicarbonate levels. As lung volumes decrease, airway closure phenomena during exhalation may occur, contributing to subtle changes in arterial oxygenation as pregnancy advances. Pregnant women also experience a reduced tolerance for apnea or hypoventilation, highlighting the need to maintain adequate ventilation.
In the later stages of pregnancy, "lightening" occurs as the fetus descends into the pelvis, often alleviating dyspnea and improving maternal respiratory comfort. However, during labor and delivery, the discomfort associated with these processes can lead to further hyperventilation, accentuating the respiratory system's adaptations. These intricate changes ensure the maternal body can effectively meet the oxygen and metabolic demands of pregnancy while preparing for the physiological challenges of delivery.
|
|
Value near term compared with non-pregnant state |
|---|---|
|
Minute ventilation Tidal volume Breathing frequency |
Increased 50% Increased 40% Increased 15% |
|
Lung volumes Expiratory reserve volume Residual volume Functional residual capacity Vital capacity Total lung capacity |
Decreased 20% Decreased 20% Decreased 20% No change Decreased 0-5% |
|
Arterial blood gasses and pH \(\ce{PaO2}\) \(\ce{PaCO2}\) pH |
Normal or slightly decreased Decreased 10 mm Hg No change |
|
Oxygen consumption |
Increased 20% |
Renal Changes
During pregnancy, the urinary system undergoes significant chronological changes to meet the growing physiological demands of the fetus. Early in pregnancy, blood volume and GFR increase significantly, with the GFR rising by approximately 60 percent compared to nonpregnant levels. This elevated GFR results in greater filtration of substances such as glucose and amino acids, sometimes leading to glycosuria and aminoaciduria due to limited tubular reabsorption. Concurrently, the regulation of ADH and thirst adjusts, with a lower set point for both, leading to heightened ADH secretion, increased thirst, and reduced serum osmolality, potentially influenced by hCG, though the exact mechanisms remain unclear.
By the end of the first trimester, normal creatinine levels begin to decline, falling to around 0.7 mg/dL and further decreasing to 0.5–0.6 mg/dL by term. Blood urea nitrogen (BUN) levels also drop. During this period, frequent urination is common, driven by hormonal changes such as the effects of progesterone, which reduces smooth muscle tone in the bladder and ureters. This hormonal effect causes dilation, incomplete bladder emptying, and urine stagnation, increasing the risk of ureteral obstruction, hydronephrosis, and pyelonephritis. Progesterone also promotes natriuresis, leading to sodium loss in the urine. However, this effect is counteracted by heightened renal tubular sodium reabsorption, influenced by aldosterone, estrogen, and deoxycorticosterone. Additionally, the increased GFR contributes to higher urinary excretion of substances, including glucose and critical nutrients like folate and vitamin B12, underscoring the heightened nutritional demands of pregnancy.
As pregnancy progresses, the renal calyces, pelvis, and ureters enlarge further, and urinary excretion of glucose and proteins continues to rise. Mechanical pressure from the growing uterus on the bladder exacerbates urinary frequency, which becomes particularly pronounced at term. This increased frequency is compounded by heightened urine production as the maternal system manages metabolic waste from both the mother and fetus. These changes require a delicate balance to maintain fluid homeostasis and ensure fetal growth and maternal well-being. However, the urinary system's vulnerability during pregnancy is evident, with complications such as urolithiasis occurring in approximately 1 in 200 pregnancies. These intricate physiological adjustments emphasize the dynamic nature of renal function during pregnancy and the importance of monitoring for potential complications.
Digestive Changes
Nausea and vomiting, commonly referred to as "morning sickness," are frequent discomforts during the early stages of pregnancy, affecting approximately 70 percent of women. These symptoms are driven by significant hormonal fluctuations, including increases in estrogen, progesterone, and hCG. This hormonal surge heightens sensitivity to various stimuli, particularly odors, which can easily trigger nausea. Additionally, changes in gastrointestinal motility, such as decreased intestinal peristalsis, contribute to nausea. These symptoms typically begin early in pregnancy, with relief often occurring around the 12th week, though some women experience persistent discomfort beyond the first trimester.
As pregnancy progresses, the fetus begins to rely on glucose as its primary energy source, which is transported across the placenta through facilitated diffusion. Amino acids, on the other hand, are actively transported from maternal to fetal blood because the fetal amino acid concentration is often higher than in maternal blood. This process requires energy (from ATP) to move amino acids against their concentration gradient. Ketones diffuse freely across the placental barrier, providing an alternative energy source when needed. Pregnancy is characterized as a diabetogenic state due to the effects of placental hormones such as placental lactogen, human growth hormone (somatomammotropin), cortisol, and progesterone, which increase insulin resistance. To meet the elevated metabolic demands, insulin levels rise, along with thyroid hormones that boost the metabolic rate. Other hormones, including PTH, relaxin, aldosterone, TSH, prolactin, ACTH, glucocorticoids, and gastrin, also increase, promoting amino acid mobilization, enhanced calcium regulation, and overall physiological adaptations required for fetal development and maternal well-being.
During the later stages of pregnancy, gastrointestinal changes become more pronounced due to both hormonal and mechanical influences. Progesterone reduces the tone and motility of the GI tract, leading to bloating and constipation. Constipation is exacerbated by mechanical obstruction caused by the enlarging uterus, decreased intestinal motility, and increased water absorption in the intestines. Heartburn, a common discomfort, arises from a combination of increased intra-abdominal pressure from the growing uterus, diminished competence of the lower esophageal sphincter, and delayed gastric emptying. This delayed gastric emptying can be further exacerbated by the onset of labor or the administration of opioids, increasing the risk of pulmonary aspiration in some cases.
Pregnancy also alters biliary function, increasing cholesterol saturation in bile and raising the risk of gallstone formation. Elevated levels of gastrin contribute to increased gastric volume and decreased pH, intensifying digestive discomfort. Additionally, liver enzymes, including aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, and alkaline phosphatase, are typically elevated, reflecting the metabolic adaptations required to support maternal and fetal needs.
Metabolic Changes
Pregnancy induces metabolic changes that facilitate the transfer of essential nutrients to the growing fetus. The fetus demands a readily convertible source of energy, leading to sustained high blood glucose levels after meals to facilitate placental transfer. This metabolic demand results in less glycogen being deposited in the maternal muscle and liver, allowing more glucose to circulate for an extended duration in the maternal bloodstream. The positive nitrogen balance peaks at the 28th week, with a total retention of 500 grams by the end of pregnancy, distributed equally between fetal and maternal gains. This period is characterized by an increased appetite and reduced gastrointestinal motility, contributing to improved nutrient absorption. Hormones such as hCG and placental growth hormone play a role in decreasing deamination, subsequently reducing urea levels in urine. By the 30th week of pregnancy, approximately 9 pounds of fat are stored, accompanied by an increase in blood fat levels. Notably, pregnant individuals are more susceptible to ketosis under conditions of stress, such as during labor.
Integumentary Changes
During pregnancy, hyperpigmentation manifests as various skin changes, often attributed to increased levels of estrogen. This hormonal influence contributes to the development of spider angiomas on the face, upper chest, and arms, as well as palmar erythema. Estrogen-mediated effects extend to the skin, causing an increase in eccrine sweating and sebum excretion. Additionally, stretching the skin during pregnancy may result in striae gravidarum, commonly known as stretch marks. These dermatological alterations are characteristic responses to the complex hormonal shifts that occur during pregnancy, emphasizing the multifaceted nature of physiological changes in expectant mothers.
Skeletal Changes
During pregnancy, hormonal changes, including increased levels of progesterone and relaxin, initiate adjustments in the maternal musculoskeletal and calcium regulatory systems to support fetal development and prepare the body for childbirth. Early in pregnancy, relaxin and progesterone promote the relaxation of the pubic symphysis and sacroiliac joints, facilitating childbirth biomechanics. This relaxation, however, contributes to lordosis, an excessive inward curvature of the spine, which develops as the center of gravity shifts to remain over the legs. While this adjustment is necessary for maintaining balance, it can result in gait unsteadiness and increased susceptibility to falls, often causing discomfort for expectant mothers.
Simultaneously, total serum calcium levels begin to decline due to reduced serum albumin levels, reaching their lowest point around 34–36 weeks. However, serum-ionized calcium remains constant, maintaining vital physiological functions. Pregnancy is marked by "physiologic hyperparathyroidism," characterized by increased calcium absorption in the gut and reduced renal calcium losses. These adaptations ensure adequate calcium availability without depleting maternal bone density, although the rate of bone turnover and remodeling progressively increases, reaching twice the normal rate by term. Elevated calcium reabsorption in the kidneys and increased levels of 1,25-dihydroxyvitamin D further support maternal and fetal calcium needs, ensuring critical skeletal and metabolic development.25
17.14: Maternal Changes During Pregnancy is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.
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