Introduction To Pregnancy
Bridget CoilaLast updated:
Erin L. George, MFTMedical editor
Pregnancy is the time period from conception to delivery during which a baby develops in the mother's womb. For due date calculations and milestone assessments, pregnancy is counted from the first day of the last menstrual period, which occurs about two weeks before conception. For many people, a missed menstrual period is the first sign of pregnancy. (1) Pregnancy lasts approximately 40 weeks, or just over 9 months, though any range between 38 to 42 weeks is considered normal, or full term. Babies born earlier than 37 weeks are considered premature, while babies born between 37 and 38 weeks are considered early term. (2)
The timeline of a pregnancy is separated into three trimesters, each lasting approximately 12 to 14 weeks. During the earliest stages, the developing baby changes from a fertilized egg into a ball of cells known as an embryo. At the end of eight weeks, after major organs have begun to form, the developing baby is considered a fetus. (3)
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Pregnancy can be detected through blood tests or urine tests. All pregnancy tests detect the presence of human chorionic gonadotropin (hCG), a hormone produced by the placenta that becomes detectable around 10 days after conception. Testing earlier than this can result in a false negative. Waiting to test until 1 to 2 weeks after the first missed menstrual period provides a more reliable result. (4)
Urine-based pregnancy tests simply detect whether hCG is present. Because they tend to provide rapid, accurate results, these are typically used to diagnose a pregnancy both through at-home tests and in a doctor's office. Blood tests can either detect the presence of hCG or the actual amount of hCG present in the blood sample. Quantitative tests that assess how much hCG is present can be used to diagnose anomalies, including the presence of twins, potential miscarriages, or ectopic pregnancies. (5)
The start of labor marks the beginning of the birthing process and the end of pregnancy. Labor may progress quickly and be over in less than an hour or may continue for days before the baby is born. There are three stages of labor:
Early labor begins with contractions, which are typically irregular at first. A clear or pink discharge may appear as the mucus plug sealing the cervix is released. As contractions become more regular and the cervix begins to dilate, early labor shifts into active labor. As the cervix dilates to approximately 10 centimeters in diameter and contractions become closer together, this signals the final part of active labor, called transition.
After about 15 to 60 minutes of transition, the birth process begins, and the baby emerges into the world headfirst. The process of birth can take anywhere from a few minutes to a few hours.
After the birth of the baby, the parent's body releases the placenta. This process may take up to an hour and usually includes milder contractions than experienced during the earlier stages of labor. (6)
The first trimester of pregnancy includes everything from the first day of the last menstrual period to the 12th week of pregnancy. During the first trimester, the fertilized egg develops into an embryo, which implants into the uterine wall. The amniotic sac, placenta, and umbilical cord form during this trimester. By the end of the first trimester, the fetus is approximately .5 to 1 ounce and 3 to 4 inches long. (7)
During this stage of pregnancy, the pregnant individual may experience symptoms of pregnancy, including nausea, mood swings, swollen breast tissue, and fatigue. The developing fetus is also particularly susceptible to external influences that can hamper development, such as exposure to alcohol, drugs, medications, environmental toxins, infectious diseases, and radiation.
The second trimester is the period from the 13th week to the 28th week of pregnancy. At this point, the placenta takes over much of the hormone production previously handled by the corpus luteum lining the uterus. This switch typically results in a reduction of traditional early-pregnancy symptoms, including morning sickness and fatigue. (8)
During the second trimester, visible signs of pregnancy start to develop, including growth of the belly and breasts. New symptoms may emerge, including leg cramps, Braxton Hicks contractions, increased vaginal discharge, and dental sensitivity. Many pregnant people experience reduced fatigue and increased excitement about the upcoming birth, while some individuals may develop anxiety related to birth and parenting. (9)
The third trimester of pregnancy includes everything from the 29th week of pregnancy through birth, which usually occurs at around 40 weeks. At around week 36, the fetus flips to face head-down in preparation for birth. By approximately 38 weeks, the fetal lungs have developed fully. At birth, the fetus usually weighs between 6 and 9 pounds and measures around 19 to 21 inches. (10)
The growing fetus puts pressure on other organs in the mother's body during the last trimester. This often results in an increased need to urinate, more frequent heartburn, difficulty taking deep breaths and frequent backaches. Fluid retention can cause swelling, particularly in the hands, face, and ankles. The discomfort of late pregnancy can take a toll on mental and emotional health, but this is usually temporary, since these symptoms typically go away after birth.
Some women experience a condition called postnatal depression after birth that makes it difficult to care for the newborn. Depression may also develop during pregnancy itself, a condition called perinatal depression. Symptoms of perinatal or postpartum depression include feelings of sadness, hopelessness, anxiety, irritability, and fatigue to the point of being unable to carry out self-care tasks. Treatment may be necessary, especially if these symptoms persist after the birth. (11)