Which of the following statement is true about the anterior fontanelle?

Size of anterior fontanelle is 2.5 cm x 2.5 cm. It closes by 18 months. It must be measured across the borders. Delayed closure is seen in hypothyroidism, rickets, trisomy syndromes, intrauterine infections, hypophosphatasia and osteogenesis imperfecta. Ref: Clohey, 8th edition, pg no 103; Dutta Obstetrics, 8th edition, pg no 95