page_title: Pregnancy
section_title: Maternal changes
hierarchical_section_title: Pregnancy / Physiology / Maternal changes
caption_attribution_description: English: A pregnant woman, pregnancy in the third trimester This photograph was taken with a Olympus E-M1.
context_page_description: Pregnancy, also known as gestation, is the time during which one or more offspring develops inside a woman. A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but can occur through assisted reproductive technology procedures. A pregnancy may end in a live birth, a spontaneous miscarriage, an induced abortion, or a stillbirth. Childbirth typically occurs around 40 weeks from the start of the last menstrual period. This is just over nine months – where each month averages 31 days. When using fertilization age it is about 38 weeks. An embryo is the developing offspring during the first eight weeks following fertilization, after which, the term fetus is used until birth. Signs and symptoms of early pregnancy may include missed periods, tender breasts, nausea and vomiting, hunger, and frequent urination. Pregnancy may be confirmed with a pregnancy test.
  Pregnancy is divided into three trimesters, each lasting for approximately 3 months. The first trimester includes conception, which is when the sperm fertilizes the egg.
context_section_description: During pregnancy, a woman undergoes many physiological changes, which are entirely normal, including behavioral, cardiovascular, hematologic, metabolic, renal, and respiratory changes. Increases in blood sugar, breathing, and cardiac output are all required. Levels of progesterone and estrogens rise continually throughout pregnancy, suppressing the hypothalamic axis and therefore also the menstrual cycle. A full-term pregnancy at an early age reduces the risk of breast, ovarian and endometrial cancer and the risk declines further with each additional full-term pregnancy.
  The fetus is genetically different from its mother, and can be viewed as an unusually successful allograft. The main reason for this success is increased immune tolerance during pregnancy. Immune tolerance is the concept that the body is able to not mount an immune system response against certain triggers.
  During the first trimester, minute ventilation increases by 40%. The womb will grow to the size of a lemon by eight weeks. Many symptoms and discomforts of pregnancy like nausea and tender breasts appear in the first trimester.
  During the second trimester, most women feel more energized, and begin to put on weight as the symptoms of morning sickness subside and eventually fade away. The uterus, the muscular organ that holds the developing fetus, can expand up to 20 times its normal size during pregnancy.
  Final weight gain takes place during the third trimester, which is the most weight gain throughout the pregnancy. The woman's abdomen will transform in shape as it drops due to the fetus turning in a downward position ready for birth. During the second trimester, the woman's abdomen would have been upright, whereas in the third trimester it will drop down low. The fetus moves regularly, and is felt by the woman. Fetal movement can become strong and be disruptive to the woman. The woman's navel will sometimes become convex, "popping" out, due to the expanding abdomen.
  Head engagement, where the fetal head descends into cephalic presentation, relieves pressure on the upper abdomen with renewed ease in breathing. It also severely reduces bladder capacity, and increases pressure on the pelvic floor and the rectum.
  It is also during the third trimester that maternal activity and sleep positions may affect fetal development due to restricted blood flow. For instance, the enlarged uterus may impede blood flow by compressing the vena cava when lying flat, which is relieved by lying on the left side.
caption: Pregnant woman in third trimester of pregnancy (last month)
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