Pregnancy
文摘
科学
2024-07-04 07:00
澳大利亚
If you would like to view past exciting content, please click on the following link.Pregnancy is a remarkable process that impacts nearly all body systems.During pregnancy, levels of estrogen and progesterone gradually increase, leading to various anatomical and physiological changes throughout the body.The journey begins with ovulation, which we can designate as day 0.On this day, within the ovary, an ovarian follicle – consisting of an egg or oocyte and its surrounding tissues – matures, and ovulation takes place. This is when the egg is released, and the surrounding structure transforms into the corpus luteum, which promptly begins to produce estrogen and progesterone.Typically, the egg is fertilized by a sperm within 12-24 hours, forming a zygote, so we can assume fertilization occurs on day 1.Shortly thereafter, cellular division occurs repeatedly until a cluster of cells known as the blastocyst forms by day 4.The blastocyst then drifts within the uterus for another day before it attaches itself to a specific site in the uterus on day 5.During this period, the corpus luteum significantly increases progesterone production relative to estrogen. This lower estrogen to progesterone ratio is crucial for the implantation process.At this stage, the blastocyst comprises two components: an inner cell mass that will develop into the fetus, and an outer layer of cells called the trophoblast, which embeds into the endometrium on day 6 and eventually becomes the fetal part of the placenta.By day 8, the trophoblast cells begin to secrete human chorionic gonadotropin (HCG), a hormone essential for two reasons.Firstly, it signals the corpus luteum that implantation has successfully occurred in the endometrium, instructing it to continue producing estrogen and progesterone.Secondly, HCG is the hormone detected by most pregnancy tests, which can indicate a positive result as early as day 9.If HCG were not to increase on day 8, the corpus luteum would begin to deteriorate by day 10, causing a decline in estrogen and progesterone levels.This decline would lead to the shedding of the endometrial lining, resulting in menstruation.A full-term pregnancy lasts about 40 weeks, approximately 9 months, counting from the last menstrual period, which typically occurs around 2 weeks before day 0 of ovulation.Thus, if counting from day 0, a pregnancy spans approximately 38 weeks.The additional two-week period is included because while women generally know when their last menstrual period started, they typically don't know the exact timing of ovulation.During the first trimester, which spans weeks 1 through 13, the corpus luteum predominantly produces hormones such as estrogen and progesterone.By around week 9, levels of HCG reach their peak before starting to decline, signaling the corpus luteum to begin degenerating.Fortunately, as the corpus luteum diminishes, the placenta takes over. Specialized trophoblast cells called syncytiotrophoblasts begin producing progesterone and estriol, the predominant form of estrogen during pregnancy.Additionally, the placenta produces small amounts of HCG and another hormone called human placental lactogen (hPL), which moderates the effects of maternal insulin to maintain high glucose levels in the blood for the fetus.Many physiological changes during pregnancy are associated with the growth of the uterus.Originally a pelvic organ, the uterus expands into the abdomen during pregnancy, reaching the level of the umbilicus by the 20th week and the xiphoid process by the 36th week.The fundal height, which measures the distance from the symphysis pubis to the top of the uterus (the fundus), provides a useful estimate of gestational age. For instance, at 36 weeks, it is typically about 36 cm, whereas at 20 weeks, it is around 20 cm.To support the needs of the mother, the enlarging uterus, and the growing fetus, and to have reserves for blood loss during delivery, the cardiovascular system must expand.Pregnancy is characterized as a high volume state because the total blood volume increases by 30-50%, leading an average woman's blood volume to rise from about 5 liters to approximately 7.5 liters by the third trimester.Although there is a slight increase in red blood cells, the plasma volume— the part of the blood without red cells—increases significantly, causing the hematocrit, or the proportion of blood composed of red blood cells, to decrease. This is known as the "physiological anemia of pregnancy."To circulate this additional volume of blood, the heart rate increases by about 20 beats per minute, boosting cardiac output.The heart mildly enlarges due to the increased workload, but this enlargement reverses post-pregnancy.This high volume state may also explain the presence of a third heart sound or physiological S3, along with a split S1, where the mitral valve closes slightly before the tricuspid valve.Despite the increased volume of blood being pumped, blood pressure slightly decreases because progesterone causes the blood vessels to dilate.As the uterus expands, it exerts upward pressure on the diaphragm, slightly elevating the heart and causing a minor leftward shift in the heart's point of maximum intensity—the location where the heart's pulsations are most strongly felt against the chest wall.Additionally, the enlarging uterus compresses the pelvic veins, which can lead to the pooling of blood, resulting in varicose veins and swelling in the lower legs and ankles.When lying flat, the uterus can exert pressure on the inferior vena cava, reducing the return of blood to the right atrium and potentially causing hypotension. Adopting a side-lying position or using a pillow to elevate the hip can mitigate this effect.The increased cardiac output enhances renal blood flow, leading to an elevated glomerular filtration rate and greater urinary output. The enlarged uterus also presses directly on the bladder, contributing to the frequent need to urinate observed in pregnant women.Moreover, the kidneys adapt to this increased demand by enlarging, causing the calyces and renal pelvis to dilate—a condition known as physiological hydronephrosis—and the ureters to widen, resulting in physiological hydroureter. Progesterone contributes to the hypomotility of the ureters, which, combined with increased urine retention and stasis, heightens the risk of upper urinary tract infections.The respiratory system is also affected. The diaphragm, pushed upward by the uterus, restricts lung expansion, making breathing more difficult. However, progesterone induces the relaxation of thoracic ligaments, increasing the rib cage's transverse and anteroposterior diameters—mechanisms akin to a swinging bucket handle and a lifting handle, respectively. These changes augment both the tidal volume (air volume per breath) and minute volume (total air volume exchanged per minute), leading to a decrease in blood carbon dioxide levels and a mild respiratory alkalosis, which actually improves gas exchange across the placenta, aiding fetal oxygenation.In the upper respiratory tract, increased estrogen levels cause more blood vessel formation and capillary engorgement, leading to nasal stuffiness, sinus congestion, and nosebleeds.Additionally, progesterone and relaxin, another hormone produced by the placenta, relax the ligaments around the sacroiliac joints and symphysis pubis to facilitate the fetal passage during childbirth. This relaxation contributes to the characteristic waddling gait often observed in pregnancy and may also induce pain in other joints, such as those in the ribs and coccyx, due to shifting ligaments.In the gastrointestinal system, hormonal changes during pregnancy lead to the relaxation of smooth muscles and a decrease in peristalsis, often causing constipation and bloating. Stool softeners may alleviate these symptoms.These hormonal changes also relax the lower esophageal sphincter, increasing the risk of gastric reflux and heartburn. Proton pump inhibitors, such as omeprazole, are effective in managing these conditions.Many pregnant women experience "morning sickness," which involves nausea not only in the morning but potentially at any time of the day, and it may persist throughout the pregnancy. Pyridoxine, or vitamin B6, can help alleviate nausea. If ineffective alone, a combination of doxylamine succinate and pyridoxine may be beneficial.Pregnancy can also alter taste preferences, leading some women to dislike certain foods while craving others. An extreme example is pica, characterized by cravings for non-food items such as ice, dirt, or starch, though the cause of this is not well understood.Fluctuations in estrogen and progesterone levels can significantly impact mood, ranging from mild irritability to severe anxiety and depression, which can be both unexpected and distressing.Cognitive changes, often described as mental fogginess or reduced concentration, may be attributed to hormonal shifts, fatigue, or sleep deprivation.Estrogen and progesterone also stimulate breast development and prepare the milk-producing structures for activity post-delivery. Increased blood flow and tissue growth in the breasts can cause sensations of tingling, fullness, and tenderness.Estrogen promotes the secretion of prolactin from the anterior pituitary gland, essential for milk production. However, the high progesterone levels during pregnancy inhibit the milk letdown effect until after birth, when estrogen and progesterone levels decrease.The anterior pituitary gland also releases more melanocyte-stimulating hormone, leading to the darkening of the areolae and the linea alba (white line). The latter transforms into the linea nigra, a dark line running down from the xiphoid process to the symphysis pubis.Pregnancy increases thyroid hormone output to boost the basal metabolic rate, meeting the heightened demands of pregnancy.Estrogen also enhances blood clotting by increasing plasma fibrinogen levels, coagulation factor activity, and platelet aggregation, while simultaneously decreasing antithrombin III activity. This hypercoagulable state reduces bleeding post-delivery but increases the risk of venous thromboembolism, particularly in the lower legs where venous stasis is common.Typically, women gain 25 to 35 pounds during pregnancy, attributed mainly to increased blood volume, the fetus, fat reserves, the uterus, and the placenta.The additional weight and forward shift in the center of gravity can cause lordosis and lower back pain. Pressure from the uterus may also lead to diastasis recti, where the rectus abdominis muscles separate, further complicating physical comfort and sleep.To summarize, pregnancy, also known as gestation, is the period during which one or more offspring develop inside a woman. Following the fertilization of an ovum by sperm, the fertilized ovum begins dividing and eventually develops into a fetus. This developmental period, known as pregnancy, typically spans 40 weeks in humans. Throughout this time, significant increases in the hormones estrogen and progesterone induce numerous changes across virtually every organ system. These changes include an increased blood volume, heightened urinary output, shallower breathing, mood fluctuations, nausea, alterations in taste preferences, skin darkening, breast modifications, and the relaxation of ligaments. These physiological adjustments are all crucial in preparing the body to deliver a healthy baby."Medical Physiology" Elsevier (2016)"Physiology" Elsevier (2017)"Human Anatomy & Physiology" Pearson (2018)"Principles of Anatomy and Physiology" Wiley (2014)"Multiple-micronutrient supplementation for women during pregnancy" Cochrane Database of Systematic Reviews (2019)"Constipation, haemorrhoids, and heartburn in pregnancy" BMJ Clin Evid (2010)"Inducing Tolerance to Pregnancy" New England Journal of Medicine (2012)"Accuracy of single progesterone test to predict early pregnancy outcome in women with pain or bleeding: meta-analysis of cohort studies" BMJ (2012)"ACOG Committee Opinion No. 343: psychosocial risk factors: perinatal screening and intervention" Obstet Gynecol (2006) - - - - - - - END - - - - - - -
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德国精英大学非精英医学博士，
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