Fetal Growth and Development

Published on 21/04/2015 by admin

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Chapter 2

Fetal Growth and Development

Fetal growth assessments can be made clinically by assessing the fundal height; clinical assessment of fetal weight can be made by performing Leopold maneuvers ( Fig. 2-1).

Fundal height is measured from the upper edge of the symphysis pubis to the top of the uterine fundus. Between 20 and 34 weeks of gestation, fundal height measurements (in centimeters) approximate the gestational age (in weeks). A discrepancy between measured and expected fundal height measurements of 3 centimeters or more is suggestive of fetal growth restriction.

Leopold maneuvers involve the palpation of the fetus through the maternal abdomen. Advantages of Leopold maneuvers include the fact that the procedure is relatively easy to perform and does not incur the expense of ultrasound; disadvantages include a low sensitivity for macrosomia. In general, clinical estimates of fetal weight are more likely to underestimate the weight of macrosomic infants than to overestimate the weight.

Ultrasound is generally used to evaluate possible fetal growth abnormalities. Biometric measurements used to assess fetal growth are as follows:

When a single sonographic measurement is used, the BPD or FL is generally the most reliable indicator of fetal age, whereas the AC is the most sensitive indicator of fetal growth.

When fetal growth is estimated, several individual biometric parameters are commonly entered into a standard formula to calculate a composite weight. Because two-dimensional estimates of fetal weight do not account for variation in fetal body composition and because of the margin of error inherent in sonographic measurement of fetal biometries, sonographic assessments of fetal weight are associated with a significant (~10% to 20%) margin of error.

Macrosomia is a term used to describe excessive fetal growth. No threshold weight has been universally accepted, but common definitions include a birth weight above 4000 or 4500 grams. In contrast to macrosomia, which is determined solely by birth weight, the term large for gestational age is used to describe any fetus with an estimated weight above the 90th percentile for a given gestational age.

Because of the pejorative nature of the term retardation, the term restriction has been substituted. Intrauterine growth restriction (IUGR) is a deviation in the rate of growth of a fetus that is less than its genetically predetermined growth potential. Prenatally, intrauterine growth restriction is often defined as an estimated fetal weight that is less than the 10th percentile for a given gestational age.

Both IUGR and SGA refer to fetal growth potential. In contrast to IUGR, which is diagnosed using estimated fetal weights, SGA refers to an infant whose birthweight is below a preset weight cutoff, typically the 10th percentile for gestational age, when compared with reference population norms.

The LBW classification refers to any infant who weighs less than 2500 grams at birth, independent of gestational age. This category includes term (≥37 weeks’ gestation) SGA infants as well as premature infants who may be SGA or of appropriate size relative to their gestational age.

Factors that affect fetal growth are typically categorized as fetal, placental, or maternal in origin and are summarized in Table 2-1. Common examples include the following:

TABLE 2-1

RISK FACTORS FOR INTRAUTERINE GROWTH RESTRICTION

MATERNAL PLACENTAL FETAL
Poor or inadequate nutritional intake Mosaicism Chromosomal abnormalities
Medical disease Abnormal implantation Trisomy 13, 18, and 21
Preeclampsia Previa Turner syndrome
Chronic hypertension Accreta Genetic syndromes
Collagen vascular disease Abnormal morphology Russell–Silver
Diabetes mellitus with vascular disease Small size Cornelia de Lange
Thrombophilia (congenital or acquired) Bilobed, battledore, or circumvallate Congenital malformations
Asthma Velamentous cord insertion Anencephaly
Cyanotic heart disease Lesions Congenital heart defect
Genetic disorder Chorioangiomata Congenital diaphragmatic hernia
Environment Abruptio placentae Gastroschisis
High altitude Infarction Omphalocele
Emotional or physical stress Secondary to maternal chronic disease Renal abnormalities
Medications and drugs Chronic abruption Multiple malformations
Warfarin Infection Multiple gestation
Anticonvulsants Chorionitis Twin-twin transfusion syndrome
Retin-A Chorioamnionitis Infection
Cigarette smoking Funisitis TORCH infections: Toxoplasmosis, other (syphilis and other viruses), rubella, cytomegalovirus, and herpes simplex virus
Alcohol    
Cocaine    
Heroin    
Prior obstetric complications    
Spontaneous abortion    
Stillbirth    
Intrauterine growth restriction, low birth weight, or premature offspring    

In pregnancies at risk for IUGR, Doppler analysis is used to evaluate placental resistance and fetal status and may improve fetal and neonatal outcomes. Normal umbilical arterial Doppler flow is reassuring and rarely associated with significant morbidity. Absence of end-diastolic flow in the umbilical artery is indicative of significant placental resistance; reversal of flow is suggestive of worsening fetal status and impending demise. Abnormalities in venous circulation (e.g., ductus venosus a-wave reversal) represent worsening circulatory compromise and may reflect a greater risk of fetal death than abnormalities in the arterial circulation. §

The brain-sparing effect observed in asymmetric IUGR refers to the fetal adaptive response to chronic hypoxia, in which the fetus preferentially redistributes its blood flow to the brain, myocardium, and adrenal glands. A decreased middle cerebral artery pulsatility index may provide direct evidence of brain sparing.

Once IUGR is suspected, fetal well-being should be closely monitored with serial antenatal testing (biophysical profile ± non-stress test; Doppler studies); the frequency of testing will be influenced by the gestational age as well as the maternal and the fetal condition. The timing of delivery is based on fetal maturity, signs of fetal distress, or worsening maternal disease. §

The timing of delivery is determined by the gestational age and clinical status of the fetus. For an IUGR fetus at term or near term, delivery is indicated if fetal lung maturity has been documented, there has been minimal fetal growth observed over serial ultrasounds, significant fetal compromise is evident on testing or Doppler study, or maternal status is worsening (e.g., hypertension). The IUGR fetus is at increased risk of metabolic acidosis and hypoxia, which may be apparent in the fetal heart tracing; continuous monitoring is indicated in labor.

The PI is a widely used measurement of the infant’s relative thinness or fatness independent of race, gender, and gestational age. It is calculated from the following formula: (weight × 100)/ length3 with weight in grams and length in centimeters. Normal PI values range between 2.32 and 2.85. The PI is normal in symmetric IUGR, low in asymmetric IUGR, and high in the macrosomic fetus.

An IUGR infant is initially at risk for perinatal asphyxia, intraventricular hemorrhage, meconium aspiration, respiratory distress syndrome, impaired thermoregulation, fasting and alimented hypoglycemia, hypocalcemia, hyperviscosity–polycythemia syndrome, immunodeficiency, and necrotizing enterocolitis. The potential long-term complications are cerebral palsy, behavioral and learning problems, and altered postnatal growth.

David Barker and colleagues postulated that impaired fetal growth may be a key determinant of later development of adult diseases such as obesity, insulin resistance, type 2 diabetes mellitus, and cardiovascular disease. Poor fetal nutrition results in developmental adaptations that permanently alter subsequent postnatal physiology and thereby “program” an infant’s future predisposition to disease.

Postmaturity refers to an infant born of a post-term pregnancy, defined as a pregnancy beyond 42 weeks of gestation. Dysmaturity may occur in term or preterm infants and describes an infant who exhibits characteristics of placental insufficiency, such as loss of subcutaneous fat and muscle mass or meconium staining of the amniotic fluid, skin, and nails.