<P> The skull of a baby consists of five main bones: two frontal bones, two parietal bones, and one occipital bone . These are joined by fibrous sutures, which allow movement that facilitates childbirth and brain growth . </P> <Ul> <Li> Posterior fontanelle is triangle - shaped . It lies at the junction between the sagittal suture and lambdoid suture . At birth, the skull features a small posterior fontanelle with an open area covered by a tough membrane, where the two parietal bones adjoin the occipital bone (at the lambda). The posterior fontanelles ossify within 2 or 3 months after birth . This is called intramembranous ossification . The mesenchymal connective tissue turns into bone tissue . </Li> <Li> Anterior fontanelle is a diamond - shaped membrane - filled space located between the two frontal and two parietal bones of the developing fetal skull . It persists until approximately 18 months after birth . It is at the junction of the coronal suture and sagittal suture . The fetal anterior fontanelle may be palpated until 18 months . In cleidocranial dysostosis, however, it is often late in closing or may never close . Examination of an infant includes palpating the anterior fontanelle . </Li> <Li> Two smaller fontanelles are located on each side of the head, more anteriorly the sphenoidal or anterolateral fontanelle (between the sphenoid, parietal, temporal, and frontal bones) and more posteriorly the mastoid or posterolateral fontanelle (between the temporal, occipital, and parietal bones). </Li> </Ul> <Li> Posterior fontanelle is triangle - shaped . It lies at the junction between the sagittal suture and lambdoid suture . At birth, the skull features a small posterior fontanelle with an open area covered by a tough membrane, where the two parietal bones adjoin the occipital bone (at the lambda). The posterior fontanelles ossify within 2 or 3 months after birth . This is called intramembranous ossification . The mesenchymal connective tissue turns into bone tissue . </Li> <Li> Anterior fontanelle is a diamond - shaped membrane - filled space located between the two frontal and two parietal bones of the developing fetal skull . It persists until approximately 18 months after birth . It is at the junction of the coronal suture and sagittal suture . The fetal anterior fontanelle may be palpated until 18 months . In cleidocranial dysostosis, however, it is often late in closing or may never close . Examination of an infant includes palpating the anterior fontanelle . </Li>

When does a baby's posterior fontanelle close