Chapter 23: Pre- & Postnatal Growth & Neonatal Development

The chapter provides a comprehensive overview of human development, spanning pre- and postnatal life, noting that pregnancy averages 266 days postfertilization, often tracked clinically as 40 weeks from the last menstrual period, referred to as 'menstrual age'. Prenatal life is divided into the embryonic phase, which is categorized using the Carnegie system (ending at stage 23, approximately 53-58 days postfertilization), and the fetal period. Advances in medical imaging, such as routine ultrasound and ultrafast MRI, are essential for monitoring development and diagnosing anomalies. Ultrasound allows for dating pregnancy using measurements like crown-rump length (CRL) in the first trimester, and specialized biometry like biparietal diameter (BPD) and abdominal circumference in the second trimester. The clinical timeline includes the perinatal period (postmenstrual week 24 up to 7 days after birth) and the neonatal period (birth to 28 days postnatally), with classifications such as preterm (lesser than 37 weeks) and post-term (greater than 42 weeks) deliveries. Throughout development, growth is defined as a velocity of change in size, characterized by distinct phases: very rapid prenatally and in infancy, steady in childhood, and rapid again during the adolescent growth spurt. Documentation of growth utilizes charts based on longitudinal or cross-sectional data, which plot size increments or growth velocity. Development follows a pattern of allometric growth, meaning different body parts grow at varying proportional rates, evidenced by the relative decrease in head size compared to the body trunk and limbs from embryo to adult. The mode of delivery, whether vaginal or via Caesarean section, influences the neonate’s transition to extrauterine life, specifically affecting the early establishment of the gut microbiome, as exposure to maternal flora during vaginal birth aids gut maturation. Furthermore, birth characteristics, such as low birth weight or thinness at birth, have been prospectively linked to increased risk factors for chronic cardiovascular and metabolic diseases in adulthood, a concept known as the Fetal Origins of Adult Disease hypothesis.