Pregnancy

• 1. Introduction

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  Pregnancy begins, by the standard medical count, two weeks before conception ever happens. A woman is described as pregnant before any sperm has met any egg, because the clock starts at the beginning of her last menstrual period. This is the strange arithmetic of gestational age. Childbirth typically arrives around 40 weeks from that starting point, a span of just over nine months. Counting instead from fertilization, the length is about 38 weeks. About 213 million pregnancies occurred in 2012, the overwhelming majority in the developing world. Behind that single number sits a process of astonishing precision and risk. How does a fertilized egg, smaller than a speck, become a fetus that can dream and feel pain? Why do doctors prefer a baby to wait until the 39th week? And how did pregnancy, the most universal of human experiences, become in some places more dangerous from violence than from any obstetric cause?

• 2. The Vocabulary Of Counting

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  Gravida comes from the Latin word meaning heavy, and a pregnant woman is sometimes called a gravida. Gravidity counts how many times a woman has been pregnant. Parity counts something different: the number of times she has carried a pregnancy to a viable stage. A woman who has never been pregnant is a nulligravida, while one pregnant for the first time is a primigravida. During a second pregnancy she would be described as gravida 2, para 1, becoming gravida 2, para 2 once she delivers. Twins and other multiple births still count as one pregnancy and one birth. The terms preterm and post-term have largely replaced the older words premature and postmature. That older language described the infant's size and state of development. The newer language describes the stage of the pregnancy itself, a small shift in focus that moved attention from the baby's body to the calendar.

• 3. From Zygote To Fetus

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  The sperm and the egg unite in one of the two fallopian tubes, after the egg is released from one of the female's two ovaries. The fertilized egg, called a zygote, then travels toward the uterus, a journey that can take up to a week. Cell division begins roughly 24 to 36 hours after the two cells meet. Implantation occurs on average 8 to 9 days after fertilization. For the first roughly ten weeks of gestation, this growing mass of cells is an embryo, and the process is called embryogenesis. Cells differentiate into the various body systems, and the basic outlines of organs and the nervous system are established. By the end of the embryonic stage, fingers, eyes, mouth, and ears become visible. The placenta and umbilical cord form to connect the embryo to the uterine wall for nutrient uptake, waste removal, and gas exchange. After about ten weeks of gestational age, which is eight weeks after conception, the embryo becomes a fetus. At this stage a fetus is about 30 mm long, its heartbeat visible on ultrasound, and it makes involuntary motions. The risk of miscarriage drops sharply at this point. Sex organs begin to appear during the third month.

• 4. When The Brain Wakes

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  Electrical brain activity is first detected at the end of week 5 of gestation. This is primitive neural activity, the same kind seen in brain-dead patients, rather than the beginning of conscious thought. Synapses do not begin to form until week 17. Neural connections between the sensory cortex and the thalamus develop as early as 24 weeks. Their function, though, does not appear until around 30 weeks. That is when minimal consciousness, dreaming, and the ability to feel pain emerge. Quickening, the moment a woman first feels the fetus move, belongs to the second trimester even though the fetus has been moving since the first. It typically happens in the fourth month, around the 20th to 21st week. A woman who has been pregnant before may feel it by the 19th week. Many women do not feel movement until much later, a reminder that the same biological event arrives on no fixed schedule.

• 5. A Body Remade

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  A full-term pregnancy before the age of 25 reduces the risk of breast, ovarian, and endometrial cancer, and the risk falls further with each additional full-term pregnancy. The pregnant body undergoes changes that are behavioral, cardiovascular, hematologic, metabolic, renal, and respiratory. During the first trimester, minute ventilation increases by 40 percent. The womb grows to the size of a lemon by eight weeks. The fetus is genetically different from its mother, making it an unusually successful allograft, a transplant the body does not reject. Increased immune tolerance is the main reason it survives, preventing the mother from mounting an immune response. In the third trimester, the enlarged uterus can compress the vena cava when a woman lies flat, restricting blood flow, a problem relieved by lying on the left side. Most weight gain happens during this final stretch. Head engagement, also called lightening or dropping, occurs as the fetal head descends. It eases breathing but reduces bladder capacity, increasing the need to urinate. In a first pregnancy it may come a few weeks before the due date, or not until labour begins.

• 6. The Case For Waiting

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  Babies born between 39 and 41 weeks of gestation have better outcomes than those born either before or after that window. This stretch is called full term, and waiting for labour to begin on its own within it is best for mother and baby. At 28 weeks, more than 90% of babies can survive outside the uterus with high-quality care, though many will face heart and respiratory problems and long-term developmental disability. Events before 37 weeks are preterm and carry a range of complications. Even a planned birth before 39 weeks raises risks, from underdeveloped lungs to feeding problems caused by an underdeveloped brain to jaundice from an underdeveloped liver. After 42 weeks a pregnancy is post-term, and the risk to woman and fetus climbs significantly. For this reason obstetricians usually prefer to induce labour between 41 and 42 weeks. A study of singleton live births found that childbirth has a standard deviation of 14 days when gestational age is estimated by first trimester ultrasound, and 16 days when estimated from the last menstrual period. The due date is less a deadline than the center of a wide and forgiving range.

• 7. When Pregnancy Goes Wrong

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  Miscarriage is the most common complication of early pregnancy, with about 80% occurring in the first 12 weeks. Around 10% to 15% of recognized pregnancies end this way, and chromosomal abnormalities underlie about half of cases. Stillbirth, defined as fetal death after 20 or 28 weeks depending on the source, leaves a baby born without signs of life. Each year about 21,000 babies are stillborn in the United States. Sadness, anxiety, and guilt may follow either loss, and fathers may grieve as well. In 2016, complications of pregnancy resulted in 230,600 maternal deaths, down from 377,000 in 1990. The common causes are bleeding, infections, hypertensive diseases, obstructed labour, and pregnancy with abortive outcome. More than 20 million women each year experience ill health from pregnancy, sometimes permanently. Thromboembolic disorders, blood clots arising from the hypercoagulability of pregnancy, are the leading cause of death in pregnant women in the United States. The body's own adaptation, meant to prevent postpartum bleeding, can turn against it.

• 8. Risk, Disparity, And Danger

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  Pregnant women or those who have recently given birth in the United States are more likely to be murdered than to die from obstetric causes. These homicides combine intimate partner violence and firearms, and health authorities have called the violence a health emergency for pregnant women. They add that such deaths are preventable when providers identify women at risk and offer help. Racial imbalances run deep in pregnancy and neonatal care. The death rate for African American babies is nearly double that of white neonates, and despite falling average rates, these disparities have persisted and grown. Teenage pregnancy carries its own burdens. The WHO defines adolescence as ages 10 to 19, and adolescents face higher health risks than women who give birth at 20 to 24. Their infants face higher risk of preterm birth, low birth weight, and severe neonatal conditions, and female adolescents are often in abusive relationships at the time of conceiving. The total fertility rate in 2024 was estimated highest in Niger at 6.64 children per woman and lowest in South Korea at 1.12, a gap that shows how unevenly the experience of pregnancy is distributed across the world.