Physiological Changes of Pregnancy.
Human pregnancy lasts 266 days from fertilization or 280 days from the first day of the last menstrual period on average. Because menstrual cycles vary from individual to individual the length of pregnancy will vary and it is not possible to predict the exact date of birth. In this discussion we will use gestational dates as this is commonly used by clinicians. Pregnancy is a time of profound physiological alterations that ensure that the embryo/fetus develops and grows normally and allows for the necessary maternal accommodations to provide for wellbeing. These maternal alterations in normal physiology will cause great changes in the body which are perceived by the mother and often cause uncomfortable symptoms and changes in mood and emotions. By understanding the facts regarding the physiology of normal pregnancy, individuals are better able to cope with what is happening to their bodies. Sometimes the normal changes can become unbalanced leading to complications. By seeking prenatal care early in pregnancy these complications can be treated quicky and safely. It is also the reason why it is important to discuss any concerns with a well-educated and certified provider- an obstetrician, family doctor with obstetrical certification or midwife. Here we discuss common alterations in physiology.
Reproductive Tract Alterations
The most obvious change is the enlargement of the uterus. Many announce their pregnancy to non-family by revealing their baby bump. The non-pregnant uterus, a smooth muscle organ is a relatively small pelvic organ in a young adult who has never been pregnant, weighing about 30 grams or about one ounce. In other women it weighs about 60-70 grams or two to two and a half ounces. At the end of pregnancy, it weighs roughly 900 grams or two pounds. This is tremendous growth and represents a change of 500 t0 1000X from the nonpregnant size. The uterus grows by a combination of proliferation of new muscle cells and by an increase in the size of individual muscle cells, called hypertrophy. By 12 weeks of gestational age the uterus containing the fetus, placenta and amniotic fluid fills the pelvic cavity and begins to extend into the abdomen. By 20 weeks it reaches the umbilicus (belly button) and at 36 weeks it reaches the xiphoid process of the sternum (a tiny bone in the center of the chest and part of the rib cage). The uterus also becomes more vascular. The increase in blood vessels makes the lower part of the uterus, the cervix a bit blue in appearance. Alterations in the connective tissue of the cervix make this portion of the uterus softer to touch or palpation. The cervix also increases in size. As the uterus enlarges it pushes on the bladder, intestines, kidneys and other internal organs, leading to urinary frequency, and sometimes constipation, as well as lower back discomfort.
The uterus must be kept from contracting during pregnancy as it grows to ensure the fetus is protected from preterm labor. The cervix must change to become soft and pliable so that when labor starts it will dilate or open to allow birth, however it must stay closed until the fetus is mature. There is a very delicate balance that occurs which involves many interactions of various hormones and the extracellular matrix, comprised of many proteins like collagen and glycoproteins. How this balance is maintained is complex story involving biochemistry and physics. Suffice it to say that if this balance becomes out of whack, preterm birth might happen.
Most people understand that amenorrhea (cessation of menses) occurs and is a sign of pregnancy. Many know that the breasts begin to enlarge and may feel tingly. The skin of the breast areola becomes darker, as does the skin of the vulva. A dark line often appears in the middle of the abdomen from the umbilicus to the pelvic bone, This line is called the Linea Nigre. The skin of the face may darken causing the so called “mask of pregnancy” which is known clinically as melasma. In the beginning of pregnancy nausea and sometimes vomiting may occur which usually disappears about the 14th weeks.
It is essential to appreciate that the many physiological alterations of pregnancy occur because of the changes in the hormonal milieu. For this reason, the alternations in the endocrine system will be presented first followed by those in the cardiac and circulatory, lung and respiratory, renal, hematological, and gastrointestinal systems.
Endocrine Alterations in Pregnancy
Human chorionic gonadotropin or hCG is produced by the trophoblasts (a component of the placenta) very early in pregnancy and maintains the corpus luteum of the ovary to prevent ovulation. As noted in our previous blog, the presence of this hormone in blood and urine is an early marker of pregnancy. By 9 to 10 weeks of gestational age the placenta secretes estrogen and progesterone in fairly large amounts. Progesterone is responsible for maintaining a quiet, noncontractile uterine myometrium, consisting of three layers of smooth muscle. The placenta also secrets hPL or placental lactogen. The placental hormonal secretions stimulate the thyroid and the release of thyroid hormones. The thyroid gland enlarges and develops an increased vasculature. Because the liver hepatocytes produce more thyroid binding hormone the amount of free thyroid hormones, the active hormones do not change levels allowing for maternal thyroid homeostasis. hPL has a structure like insulin and therefore its secretion alters glucose metabolism. While this is important in providing the increased metabolic needs of pregnancy, this increase may lead to gestational diabetes. Corticotropin releasing hormone or CRH stimulates maternal adrenal corticotropic hormone. Melanocyte stimulating hormone increases leading to the darkening of skin areas, such as the vulva and the breast areolar areas. The pituitary gland increases in size by - up to 135%. Prolactin, a pituitary hormone, important for lactation increases by ten-fold. Relaxin is another hormone secreted in pregnancy by the decidua (endometrium of pregnancy). It promotes pregnancy maintenance. It enhances insulin action, so its absence is detrimental to glucose metabolism. However, too much relaxin is associated with preterm birth.
Cardiovascular Alterations
At 6 weeks gestational age the amount of blood pumped out of the heart, the cardiac output (CO) starts to increase. This will increase to 30-50% of the prepregnant output by its peak at 30 weeks of pregnancy and then stabilizes. The cardiac output increases another 30% at labor or parturition. Cardiac stroke volume increases. The blood flow to the uterus increases to 1Liter a minute to ensure adequate uteroplacental circulation. In addition, this increase in cardiac output is necessary to allow the maternal kidneys to excrete both maternal and fetal waste products. Circulation to the skin is also increased to help regulate body temperature. The maternal heart rate increases from the nonpregnant state. An individual with a heart rate of 70 beats per minute will have a heart rate of 90 beats per minutes. Blood pressure, however, decreases or drops in the second trimester (12 to 20 weeks). All these physiological changes must be completely understood by the health care provider, especially the fact that blood pressure decreases. Thus, a normal blood pressure reading in a nonpregnant individual is high for a pregnant person. Another important consideration is that after about 30 weeks the cardiac output becomes sensitive to maternal position. For instance, lying down in a supine position (on the back) causes pressure on the vena cava by the large pregnant uterus, decreasing blood flow especially through the placenta. Varicose veins in the lower extremities and hemorrhoids often occur. This pressure can decrease the uteroplacental blood flow. For this reason, it is recommended that lying on the left side is the preferred position during pregnancy.
Respiratory System Alterations
Progesterone signals the brain to lower carbon dioxide (CO2) levels. The respiratory rate increases. As the uterus enlarges and pushes against the diaphragm lung function is compromised. Dyspnea (shortness of breath) occurs which can cause concern in some pregnant individuals. Many pregnant individuals need to raise their head and chest to sleep well.
Gastrointestinal Alterations
Progesterone deceases intestinal motility causing delayed gastric emptying. Hydrochloric acid is decreased in the stomach as well. There usually is nausea and sometimes vomiting in the first trimester, induced by the secretions of the placental hormones. Nausea usually disappears in the by 14 weeks. However, in some the nausea is severe and does not change and vomiting and dehydration occurs which needs to be treated, often with intravenous fluids. However, this is not a common problem in most pregnancies. Because of the decreased intestinal motility and to the pressure of the enlarging uterus on the rectum and lower colon, constipation can be a problem.
Urinary Tract Alterations
Between 16 to 24 weeks gestational age the glomerular filtration rate (GFR) of the blood flow through the kidneys increased 30 to 50% and remains elevated until birth. Thus, kidney function tests results are changed during pregnancy. For instance, blood urea nitrogen (BUN) and creatine clearance rate (CCR) which measures kidney function is decreased compared to the nonpregnant state. Progesterone secretion also leads to dilation of the ureters.
Postpartum Restoration
After birth the uterus must shrink. The uterine muscles contract and the increased muscle tissue is eliminated by the body by the physiological process of involution. By 6 to 8 weeks postpartum the uterus has gone back a normal size. Breast milk begins to be secreted. In 2022 83% of babies were fed their mother’s milk at first. By 6 months of age 56% of infants were still getting mother’s milk. The first milk is called colostrum and is high in immune cells and antibodies as well as mineral and vitamins, and sugars. By two to three days after birth the milk increased in amount and composition, called transitional milk. and by four weeks the milk is mature milk. The amazing thing about human milk is that its composition can change according to the infant’s needs. For instance, if the baby is ill, the mother’s body will produce more antibodies needed by the infant. Another positive aspect of lactation is that it aids in uterine involution. In the postpartum period, menses resumes. The menstrual cycle can be delayed if the mother is feeding the infant completely on human milk. The endocrine system and other organ system must return to their nonpregnant state. We plan to discuss the details of the postpartum period in a future blog.
Final Comments
Maternal physiology is greatly altered in pregnancy and affects multiple organs. There is a very delicate balance between normal physiology and abnormal changes that lead to pregnancy complications such as preeclampsia, a serious complication involving early alterations in placentation. This problem leads to hypertension, (high blood pressure), liver abnormalities, and kidney and heart problems) and if not treated seizures and death. Another potentially serious complication is gestational diabetes, which if not treated will lead to serious problems for the fetus and newborn and serious problems for the mother. The large for gestational age infant is only one of the concerning complications of diabetes in pregnancy. Furthermore, the alterations of physiology during pregnancy must be restored during the postpartum period. Thus, it is imperative that all pregnant individuals seek prenatal, birth and postpartum care from a well- educated and experience professional who is certified in their profession. Good outcomes do not just happen. They occur when there is trust between pregnant persons and the professional person providing their care. Pregnancy is an important and emotional time in life and it must be treated seriously and respectfully. The care must occur early in pregnancy. It must be very attentive during labor and birth and must extend one year after pregnancy.
References
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Reviewed/Revised May 2021 | Modified Sep 2022