Which period of prenatal development provides the most risk of structural problems in organ development?

Red Blood Cell Indices during Prenatal and Postnatal Development

The RBC count, Hb concentration, and hematocrit (Hct) increase throughout gestation, as shown inTable 79.5. In term infants, the mean capillary hemoglobin at birth is 19.3 g/dL (seeTable 79.4). The capillary Hct has a mean of 61 g/dL. Premature infants have lower Hb levels than do full-term infants. In addition to gestational age, Hb levels are influenced by a variety of factors that must be kept in mind when analyzing the neonate with anemia or polycythemia. One important determinant is the site of sampling: Capillary Hb values are higher than peripheral venous samples, and umbilical venous Hb results are the lowest. The interval between delivery and clamping of the umbilical cord and the height of the baby relative to the placenta can significantly affect a newborn's blood volume and total RBC mass. The placenta contains about 100 mL of blood. The mean blood volume of a full-term infant is about 85 mL/kg. Early or delayed clamping of the umbilical cord alters this mean blood volume by about 10% lower or higher, respectively. The average Hb at birth is relatively unchanged; however, 48 hours later, after redistribution of plasma volume, Hb values will reflect the lower or higher red cell mass. Racial differences also occur. One study reported significantly higher Hb, Hct, and MCV in white infants compared with black infants of similar gestational ages.4 Reticulocyte counts in the cord blood of infants average 4%-5%, and nucleated RBCs are evident in most cord blood samples (40,000/µL). These findings are presumed to reflect high EPO production secondary to low oxygen retention in utero. Infants who experience placental insufficiency and intrauterine growth restriction have higher than normal EPO production and an even greater degree of erythrocytosis. The mean MCV of RBCs in the newborn is increased. The RBCs of the neonate have an increased Hb content, but the mean corpuscular hemoglobin concentration (MCHC) is comparable to that of adults.

Delayed (30-90 seconds) cord clamping (DCC) has been shown to prevent hypotension, raise hematocrit, and decrease the need for transfusions in preterm infants.102 In addition, term infants who have had delayed cord clamping have reduced iron deficiency anemia in the first year of life but have an increased risk of early jaundice.79 Thus, it has been recommended that both preterm and full-term infants undergo routine delayed cord clamping.

A new population that has also shown benefit is the neonate who has needed in utero transfusion because of red cell alloimmunization. Garabedian et al. studied a series of 72 neonates who needed in utero transfusion because of alloimmunization. Thirty-six of the neonates had DCC, and 36 neonates did not have DCC. More infants without DCC Hb had anemia at birth. The rate of transfusion, maximum level of bilirubin, the rate of intensive phototherapy, and the total duration of phototherapy were similar in the two groups. Postnatal exchange transfusions (ET) were more likely performed in the group without DCC than in the group with DCC. The interval between birth and the first transfusion was higher in the group with DCC. The authors recommended DCC with duration of 30 seconds in infants at risk for neonatal anemia because of red blood cell alloimmunization, provided that the management of jaundice is optimized.44

Fetal Monitoring Techniques

Although speculation about the nature of fetal behavior has existed since antiquity, the advent of real-time ultrasound in the early 1970s enabled modern scientific investigation of prenatal development. Visualization can reveal specific behaviors (e.g., thumb-sucking), qualitative aspects of movement (e.g., fluidity of flexion and extension), structural features of the fetus (e.g., size), and characteristics of the uterine milieu (e.g., volume of amniotic fluid). In addition, refinement of techniques to monitor fetal heart rate and its patterning, using Doppler ultrasound, has provided another important source of information regarding prenatal neural development. Doppler has also generated techniques to detect fetal motor activity without ultrasound visualization and makes it possible to measure the amount of blood flow and resistance in maternal and fetal vessels, including umbilical, cerebral, and uterine arteries. The most recent technological advance is the development of three-dimensional (3D) and so-called four-dimensional ultrasound (i.e., 3D image plus addition of a fourth dimension of real time motion) that allows visualization of details, such as fetal facial expressions and hand movements, which were not previously possible. Figure 2 presents examples of traditional two-dimensional (2D) and 3D images.

Although 3D technology holds great potential for future studies, it is yet to be implemented broadly. Almost all knowledge about human prenatal development has been generated by information from a mixture of existing ultrasound methods, including 2D ultrasound and fetal heart rate monitoring. Regardless of how sophisticated ultrasound may become in the future, the human fetus will always remain slightly beyond our actual reach.

Prenatal Diagnosis of Genodermatoses

Many genodermatoses are incompatible with survival to term or are associated with significant morbidity or even mortality after birth, making prenatal diagnosis desirable. In the early 1980s, fetal skin biopsy obtained at 19–22 weeks' EGA via ultrasound guidance became the first technique available for prenatal diagnosis of inherited skin diseases. Epidermolytic ichthyosis and generalized severe junctional EB were the first diseases diagnosed prenatally by light and/or electron microscopy of fetal skin biopsies.

As the causative genes for many genodermatoses have been discovered, DNA-based testing using material obtained from chorionic villus sampling (10–12 weeks after the last menstrual period/8–10 weeks' EGA) or amniocentesis (14–16 weeks after the last menstrual period/12–14 weeks' EGA) has largely replaced fetal skin biopsy, as these techniques can be performed earlier and pose less risk to the mother and fetus. With these approaches, the pathogenic mutation(s) must be identified in family members prior to prenatal testing.

Preimplantation genetic diagnosis allows for prenatal diagnosis before the embryo is implanted and pregnancy begins. This technique requires the use ofin vitro fertilization. One or two cells are taken from the embryo at the blastocyst (6- to 10-cell) stage. The cellular DNA is amplified using PCR and analyzed for the known family mutation(s); unaffected embryos are then selected for uterine implantation. For X-linked disorders, sex determination has been utilized both in conjunction with specific genetic analysis and to identify embryos of a particular sex for selective transfer. Preimplantation genetic diagnosis obviates the need to terminate a pregnancy with an affected fetus. However, it has several disadvantages compared to chorionic villus sampling or amniocentesis, including higher cost, technical difficulties (e.g. contamination by extraneous DNA), and a low rate of completed pregnancies.

Growth and development of the mammary glands

Prenatal growth and development of the mammary glands are independent of sex hormones and genetic sex. Until the onset of puberty, there are no differences in the male and female breast. With the onset of puberty the duct system grows and branches and the surrounding stromal and fat tissues proliferate in response to estrogens. Following estrogen priming, progesterone promotes growth and branching of the lobuloalveolar tissue, but for these steroids to be effective, prolactin, growth hormone, IGF-I, and cortisol must also be present. Lobuloalveolar growth and regression occur to some degree during each ovarian cycle. There is pronounced growth, differentiation, and proliferation of alveoli during pregnancy, when estrogen, progesterone, prolactin, and hCS circulate in high concentrations.

Prenatal development

Three germ layers give rise to all the tissues and organs of the body. The cells of each germ layer divide, migrate, aggregate, and differentiate in rather precise patterns as they form the various organ systems. The three germ layers are the ectoderm, mesoderm, and endoderm. The ectoderm gives rise to the central nervous system, peripheral nervous system, epidermis and its appendages, mammary glands, pituitary gland, and subcutaneous glands. The mesoderm gives rise to cartilage, bone, connective tissue, striated and smooth muscle, blood cells, kidneys, gonads, spleen, and the serous membrane lining of the body cavities. The endoderm gives rise to the epithelial lining of the gastrointestinal, respiratory, and urinary tracts; the lining of the auditory canal; and the parenchyma of the tonsils, thyroid gland, parathyroid glands, thymus, liver, and pancreas. The development of the embryo requires a coordinated interaction of these germ layers, orchestrated by genetic and environmental factors under the influence of basic induction and regulatory mechanisms.

Prenatal human embryologic development can be divided into three major periods: the first 2 weeks, the embryonic period, and the fetal period. The first 2 weeks of development are characterized by fertilization, blastocyst formation, implantation, and further development of the embryoblast and trophoblast. The embryonic period comprises weeks 3 through 8 of development, and the fetal period encompasses the remainder of the prenatal period until term.

The embryonic period is important because all the major external and internal organs develop during this time, and by the end of this period, differentiation is practically complete. All the bones and joints have the form and arrangement characteristic of adults. Exposure to teratogens during this period can cause major congenital malformations. During the fetal period, the limbs grow and mature as a result of a continual remodeling and reconstructive process that enables bones to maintain their characteristic shape. In the skeleton in general, increments of growth in individual bones are in precise relationship to those of the skeleton as a whole. Ligaments show an increase in collagen content, bursae develop, tendinous attachments shift to accommodate growth, and epiphyseal cartilage becomes vascularized.

Few studies have focused on the prenatal development of the glenohumeral joint. The contributions by DePalma and Gardner were essential but did not emphasize clinical correlations between the observed fetal anatomy and the pathology seen in the postnatal shoulder.11–13 Most studies on the developing shoulder have focused primarily on bone maturation, and analysis of the soft tissue structures of the developing shoulder, such as the joint capsule and the labrum, is still incomplete.

Most recently a 2018 study from Hita-Contreras et al. reported on human embryos and fetuses with focus on glenohumeral joint development (PMID 29193070).14 The investigators further contributed to the literature regarding soft tissue development of the joint including the labrum, capsule, biceps, and glenohumeral ligaments. Despite the continued reports of human glenohumeral development, there is still a need for further studies on more specific portions of the glenohumeral soft tissue anatomy including the inferior glenohumeral ligament complex (IGHLC), which has been shown to be an integral component for stability in the adult shoulder.15 The seminal studies of the fetal glenohumeral joint were completed before the role of the soft tissue structures in shoulder stability was elucidated. There is now a greater appreciation of the anatomy and biomechanics of the static and dynamic stabilizers of the glenohumeral joint and their role in shoulder stability.

Combined Pre- and Postnatal Development and Juvenile Animal Toxicity Study Designs

Pre- and postnatal development studies are routinely conducted as part of nearly all compound registration packages whereby offspring are exposed indirectly in utero and postnatally via nursing [16]. Modifying this study design to include direct dosing of the offspring as an approach to support pediatric clinical trials has been proposed by De Schepdrijver and Bailey [15] and the FDA and EMA regulatory guidance documents [7,8]. An example of this design has been presented in the public literature [14]. The use of this design to support pediatric drug development may have limited application and is generally not recommended. Since exposure to the offspring occurs both in utero and postnatally, the relevance of findings to the safety of a particular pediatric population would be difficult to ascertain, thus potentially jeopardizing risk assessment.

Central (Intrauterine) Maturational Defect

Normal intrauterine development of skeletal muscle with regard to fiber type differentiation and morphologic features is complete by the 28th week of gestation. Some infants with hypotonia at birth do not manifest any of the pathologic features of the congenital myopathies, dystrophies, or neurogenic diseases. However, the muscle biopsy shows occasional large, hypereosinophilic fibers that appear to represent type II fibers.39 An excessive number of these fibers (>2%) can be associated with mild infantile hypotonia, which is thought to represent a delay in the normal intrauterine development of these fibers. The terms cerebral hypotonia and cerebral hypoplasia have also been applied to this disorder.

Prenatal Development

Prenatal development is divided into three successive phases. The first two, when combined, constitute the embryonic stage, and the third is the fetal stage. The forming individual is described as an embryo or fetus depending on its developmental stage.

The first phase begins at fertilization and spans the first 4 weeks or so of development. This phase involves largely cellular proliferation and migration, with some differentiation of cell populations. Few congenital defects result from this period of development because, if the perturbation is severe, the embryo is lost.

The second phase spans the next 4 weeks of development and is characterized largely by the differentiation of all major external and internal structures (morphogenesis). The second phase is a particularly vulnerable period for the embryo because it involves many intricate embryologic processes; during this period, many recognized congenital defects develop.

From the end of the second phase to term, further development is largely a matter of growth and maturation, and the embryo now is called a fetus (Figure 2-2).

Reproductive Toxicity

Prenatal development studies in mice, rabbits, rats, and hamsters have shown that propylene glycol does not produce effects on developmental parameters (growth and development of pups) in doses up to 1.2–1.6 g kg−1 (highest doses tested); a subsequent study in mice only showed no developmental effects at doses up to 10 g kg−1. A multigeneration reproductive and developmental study in mice exposed to propylene glycol in drinking water showed no effects on fertility and reproduction in adult and second-generation mice nor any effects on pup survival and development in doses up to 10 g kg−1. As such, current exposures to propylene glycol are of negligible concern for reproductive or developmental toxicity in humans.

Propylene glycol at high concentrations can be used as a cryoprotectant for preservation of embryos. In vitro studies of the effects of propylene glycol (3000 mmol l−1) on mouse zygotes have shown cell membrane damage and altered pH in the zygotes, indicating a possible effect on embryonic development. No effects were noted at 1500 mmol l−1, an extremely high concentration.

Fetal Nutrient Requirements

Prenatal development can usefully be divided into two periods: the embryonic period, which covers the first 8 weeks of life, and the fetal period, which lasts from the 9th week of gestation until term. During the latter period the fetus is entirely dependent on the placenta for its supply of nutrients. The fetus has an absolute requirement for the same essential nutrients as the adult but the adequacy of supply is particularly critical during in utero life when all the structures of the body are being established. In addition, because of the particularly high demand for some strictly nonessential nutrients these may be considered as ‘conditionally essential’ if the rate of utilization exceeds the fetal capacity for de novo synthesis.

The placenta has to maintain the supply of all nutrients at a rate adequate to allow unrestricted fetal growth. It also has to provide an appropriate mix of nutrients to meet the needs of the fetus at the different stages of pregnancy. For example, in the first two-thirds of pregnancy the fetus deposits mainly protein, while in late gestation fat takes over as the dominant form of deposition (Figure 1).

The availability of individual nutrients to the fetus depends not only on the maternal dietary intake but also on the function of the placenta and the many physiological and biochemical adaptations that occur during pregnancy (Figure 2). An understanding of placental function and its interaction with diet is essential to the setting of appropriate dietary guidelines for pregnancy.