Infants and Children in Context. Tara L. Kuther

on Prenatal DevelopmentPrinciples of TeratologyTypes of TeratogensPrescription and Nonprescription DrugsAlcoholCigarette SmokingCannabisCocaineOpioidsMaternal IllnessEnvironmental HazardsMaternal CharacteristicsNutritionEmotional Well-BeingMaternal AgePaternal CharacteristicsContextual and Cultural Influences on Prenatal Care

       ChildbirthLaborMedication During DeliveryCesarean DeliveryNatural ChildbirthHome Birth

       The NewbornMedical and Behavioral Assessment of NewbornsThe Newborn’s Perceptual CapacitiesNewborn States of ArousalLow-Birthweight Infants: Preterm and Small-for-Date BabiesContextual Risks for Low BirthweightCharacteristics of Low-Birthweight InfantsOutcomes of Low-Birthweight Infants

      Prenatal Development

      Conception, the union of ovum and sperm, marks the beginning of prenatal development. Over 266 days (or 38 weeks), the human progresses from fertilization to birth. During this transformation, the zygote progresses through several periods of development, finally emerging from the womb as a neonate.

      Conception

      A woman can conceive only during a short window of time each month. About every 28 days, an ovum bursts from one of the ovaries into the long, thin fallopian tube that leads to the uterus; this event is known as ovulation (see Figure 3.1). The ovum is the largest cell in the human body, yet it is only 1/175th of an inch in diameter (about the size of the period at the end of this sentence). Over several days, the ovum travels down the fallopian tube, which connects the ovaries to the uterus, while the corpus luteum, the spot on the ovary from which the ovum was released, secretes hormones that cause the lining of the uterus to thicken in preparation for the fertilized ovum (Sadler, 2018). If fertilization does not occur, the lining of the uterus is shed through menstruation about 2 weeks after ovulation.

      A tiny sperm is fertilizing the much larger ovum.

      ©iStockphoto.com/Dr_Microbe

      Conception, of course, also involves the male. Each day, a man’s testes produce millions of sperm, which are composed of a pointed head packed with 23 chromosomes’ worth of genetic material and a long tail. During ejaculation, about 360 million, and as many as 500 million, sperm are released, bathed in a protective fluid called semen (Moore & Persaud, 2016). After entering the female’s vagina, sperm travel through the cervix into the uterus and onward toward the ovum. After about 6 hours the sperm reach the fallopian tube, where an ovum may—or may not—be present. The journey is difficult: Some sperm get tangled up with other sperm, some travel up the wrong fallopian tube, and others do not swim vigorously enough to reach the ovum. On average, about 300 sperm reach the ovum, if one is present (Webster, Morris, & Kevelighan, 2018). Those that travel up the fallopian tube can live up to 6 days, able to fertilize a yet unreleased ovum. The ovum, however, remains viable for only about a day after being released into the fallopian tube.

      Description

      Figure 3.1 Female Reproductive System

      Source: Levine and Munsch (2010, p. 102).

      Both sperm and the woman’s reproductive tract play a role in fertilization (Suarez, 2016). Sperm are guided by temperature, tracking the heat of an expectant ovum, as well as by chemical signal (Lottero-Leconte, Isidro Alonso, Castellano, & Perez Martinez, 2017). In the presence of an ovum, sperm become hyperactivated, they swim even more vigorously, and the sperm’s head releases enzymes to help it penetrate the protective layers of the ovum (Bianchi & Wright, 2016). As soon as one sperm penetrates the ovum, a chemical reaction makes the ovum’s membrane impermeable to other sperm. The sperm’s tail falls off, and the sperm’s genetic contents merge with that of the ovum.

      At the moment of conception, the zygote contains 46 chromosomes, half from the ovum and half from the sperm. After fertilization, the zygote rapidly transforms into a multicelled organism. Prenatal development takes place over three developmental periods: (1) the germinal period, (2) the embryonic period, and (3) the fetal period.

      This ball of cells, known as a morula, is formed at about 3 days after conception. Each of these cells is identical. Differentiation has not yet begun.

      Pascal Goetgheluck/Science Source

      Germinal Period (0 to 2 Weeks)

      During the germinal period, also known as the period of the zygote, the newly created zygote begins cell division as it travels down the fallopian tube, where fertilization took place, toward the uterus. About 30 hours after conception, the zygote then splits down the middle, forming two identical cells (Webster et al., 2018). This process is called cleavage, and it continues at a rapid pace. As shown in Figure 3.2, the two cells each split to form four cells, then eight, and so on. Each of the resulting cells is identical until about the third set of cell divisions. This process of cell division continues rapidly. Any of these cells may become a person (and sometimes two, in the case of monozygotic or identical twins).

      Development proceeds very quickly during the embryonic period. Note the dramatic changes from the fifth week (left) to the seventh week (right) of prenatal development.

      Petit Format/Science Source

      Cell differentiation begins roughly 72 hours after fertilization when the organism consists of about 16 to 32 cells. Differentiation means that the cells begin to specialize and are no longer identical. By 4 days, the organism consists of about 60 to 70 cells formed into a hollow ball called a blastocyst, a fluid-filled sphere with cells forming a protective circle around an inner cluster of cells from which the embryo will develop. Implantation, in which the blastocyst burrows into the wall of the uterus, begins at about day 6 and is complete by about day 11 (Moore & Persaud, 2016).

      Embryonic Period (3 to 8 Weeks)

      After implantation, by the third week after conception, the developing organism, now called an embryo, begins the most rapid period of structural development in the lifespan. All of the organs and major body systems form during the embryonic period. The mass of cells composing the embryonic disk forms layers, which will develop into all of the major organs of the body. The ectoderm, the upper layer, will become skin, nails, hair, teeth, sensory organs, and the nervous system. The endoderm, the lower layer, will become the digestive system, liver, lungs, pancreas, salivary glands, and respiratory system. The middle layer, the mesoderm, forms later and will become muscles, skeleton, circulatory system, and internal organs.

      As the embryo develops, support structures form to protect it, provide nourishment, and remove wastes. The amnion, a membrane that holds amniotic fluid, surrounds the embryo, providing temperature regulation, cushioning, and protection from shocks. The placenta, a principal organ of exchange between the mother and developing organism, begins to form. It contains tissue from both the mother and embryo and, once formed, will act as a filter, enabling the exchange of nutrients, oxygen, and wastes to occur through the umbilical cord. The placenta is also a protective barrier, preventing some toxins from entering the embryo’s bloodstream as well as keeping the mother and embryo’s bloodstreams separate. Many toxins, however, are able to pass through the placenta, including drugs and