Rather simple principles apply for the role of mechanical factors in morphogenesis. These are that the direction and magnitude of forces affect the form of the developing individual, as illustrated in Fig. 51-1 . Some of the factors that affect the magnitude and direction of forces are summarized in Table 51-1 . One major influence on the nature and alignment of forces is growth. Thus the…

Genesis The underlying mechanism for most defects related to problems associated with obstetric procedures is vascular disruption. With chorionic villus sampling (CVS), removal of villi may result in embryonic hypotension, hypoxia, endothelial damage, hemorrhage, and necrosis with tissue loss. Defects are more likely to involve distal structures in the upper body (hands and tongue) than proximal structures or the lower body, with middle digital rays more…

Genesis Constraint that occurs during the latter period of gestation can cause molded deformations with good prospects for spontaneous or assisted return to normal form; however, when constraint occurs during early morphogenesis, it can have more severe and lasting impacts on form. The types of defects that can be produced by such compression during this early period of organogenesis (i.e., within the first trimester) fall into…

Genesis Ectopic pregnancy , defined as the implantation of a fertilized ovum outside the uterus, occurs most commonly in the fallopian tube (95.5% of all ectopic pregnancies), with 70% implanting in the ampulla, 12% in the isthmus, 11% in the fimbrial end, 2% in the interstitial region, and 3% ovarian ( Fig. 48-1, A ). Only 1.3% of ectopic pregnancies are abdominal, occurring with direct implantation…

Genesis Fetal movement is essential to normal joint morphogenesis. Joints develop secondarily within the condensed mesenchyme of the developing bones, and chronic lack of movement leads to joint contractures. Fetal akinesia is associated with a specific combination of clinical findings that was previously called the Pena-Shokeir phenotype because these signs were first described as part of Pena-Shokeir syndrome. These findings include fetal growth retardation, congenital contractures…

Genesis The relative and absolute amount of amniotic fluid tends to decrease during the last trimester as the fetus fills out the uterine cavity, as shown in Fig. 46-1 . A serious deficiency of amniotic fluid will result in significant fetal constraint. Oligohydramnios may be secondary to amniotic rupture and may be accompanied by constrictive, disruptive strands of amnion ( Fig. 46-2 ). The consequences of…

Genesis Several different types of problems can limit the size and/or shape of the uterus and thus enhance the likelihood of fetal deformation. Examples include a malformed uterus caused by failure of the Müllerian ducts to completely fuse during embryogenesis, resulting in either symmetric or asymmetric structural anomalies of the uterus such as didelphic uterus with duplicated cervix, bicornuate uterus, septate uterus, and arcuate uterus (…

Genesis In face and brow presentations, the face is the compressed presenting part, usually with extension of the head ( Fig. 44-1 ). Face presentation occurs in 1 to 2 per 1250 deliveries, and persistent brow presentation is less common. Brow presentation occurs in 1 per 1444 deliveries. Anything that delays or prevents flexion such as fetal anomalies, contracted pelvis, fetopelvic disproportion or cord around the…

Genesis Transverse lie is the second most common non-vertex presentation and occurs in 12 per 1000 deliveries. It is associated with multiparity (90%), prematurity (13%), placenta previa (11%), polyhydramnios (8%), uterine anomalies (8%), and uterine myomas (3%), especially when myomas are located in the lower uterine segment. Predisposing factors such as uterine structural anomalies, prematurity, and placenta previa are found in 66% of primiparas, but only…

Genesis The frequency of singleton breech presentation at term is 3.1% and rises to 6.2% when multiple births are included. Breech presentation is an important cause of deformation, and fully one third of all deformations occur in babies who have been in breech presentation ( Fig. 42-1 ). Because 2% of newborns have deformations, this indicates that 0.6% of neonates have one or more deformations owing…

Genesis Wormian bones are accessory bones that occur within cranial suture lines, and, although they do not cause any impairment themselves, their significance is variable. The prevalence varies by size and quantity. Individuals with few and small (less than 4 mm) Wormian bones are fairly common, and individuals with many large (greater than 10 mm) Wormian bones ( Fig. 41-1 ) are extremely rare. To be considered pathologically…

Genesis Cephalohematoma is a common problem occurring in about 2.5% of newborns. It is a subperiosteal extracranial hemorrhage that may enlarge after delivery, sometimes taking weeks to resolve. This condition contrasts with the scalp edema of caput succedaneum, which reaches its maximal size at birth and usually resolves within a few days. Both lesions are believed to result from an injury to the cranial periosteum during…

Genesis Aplasia cutis congenita (ACC) is a congenital, localized absence of skin that most commonly affects the scalp. The frequency is 1 per 3000 live births and this condition can occur in isolation or as part of a heterogeneous group of syndromes. Frieden's classification system divides ACC into 9 subtypes ( Table 39-1 and Figs. 39-1 to 39-8 ). Aplasia cutis congenita begins as multiple or…

Genesis Parietal foramina are a rare calvarial defect estimated to be less than 1 in 25,000 births. They present as symmetric oval defects situated on either side of the sagittal suture near the parietal eminences and separated from each other by a narrow bridge of bone ( Fig. 38-1 ). They represent defects of calvarial ossification, with their size decreasing with advancing age. They manifest autosomal…

Genesis On very rare occasions, the anterior fontanel will ossify into a bony plate that may be slightly elevated in relation to the rest of the cranium. This may occur because of decreased growth-stretch tensile forces across the anterior fontanel and can be present at birth or appear later in infancy. It is sometimes seen with multiple suture synostosis and can also occur in otherwise normal…

Genesis Prolonged forceful pressure on the presenting part, usually at the vertex affecting the superior portions of the parietal bones, may result in diminished mineralization within the compressed region. The calvarium is generated from the underlying dura mater, and persistent growth-stretch tensile forces restrain cranial ossification and maintain sutural patency. Persistent cranial pressure mimics the mechanical impact of growth-stretch forces and results in temporarily restrained, localized…

Genesis Vertex birth molding describes the mechanical changes in fetal head shape caused by external compression on the cranium from bony adjustments within the cranial vault that occur as the neonate in vertex presentation passes through the birth canal. Additional soft tissue swelling can significantly alter the shape of the neonatal head, and pressure against the fetal cranium can delay normal ossification in the vertex region,…

Genesis Constraint as the cause of multiple sutural synostosis (sagittal, metopic, coronal, and/or lambdoid) is unusual and may only result from a profound degree of prenatal head constraint. The complete restoration of normal form after early and effective surgery is more likely when constraint is the cause of the problem ( Figs. 34-1 and 34-2 ). Multiple sutural synostosis can also result from genetic mutations in…

Genesis Lambdoidal craniosynostosis occurs in approximately 1 in 40,000 births and accounts for 2% to 4% of all cases of craniosynostosis. Previously, misdiagnosis of nonsynostotic deformational posterior plagiocephaly with occipital flattening as lambdoidal craniosynostosis resulted in apparent epidemics of craniosynostosis, in which the proportion of lambdoidal craniosynostosis cases was reported as more than 40% of total craniosynostosis cases in Colorado and more than 70% in York-Selby,…