The coronal, frontal, lambdoid and sagittal sutures and the anterior and posterior fontanelles are of greatest diagnostic value as they can be felt through the vagina during labor. It is by recognizing and locating these sutures and fontanelles at this time that the accoucheur is enabled to determine the exact position and presentation of the fetus. The fact that the skull is made up of separate bones, with soft membranous spaces interposed between them, permits of its being compressed or moulded to a considerable extent as it passes through the birth canal. Opposing margins may meet, or even overlap, to such a degree that the diameter of the head will be appreciably diminished and permit of its passage through a relatively narrow canal. This mouldability varies greatly, however, and the difference in the degree of compressibility of heads of approximately the same size may spell the difference between an easy and a difficult, or even an impossible labor. A new-born baby’s head may be so distorted and elongated by the moulding process that it is unsightly and gives the young mother great concern. But the nurse can be quite confident in her assurances that the little head will assume its normal, rounded outline in a very few days. The five most important diameters of the new-born baby’s head are: =1. The occipito-frontal= (abbreviation, O.F.), measured from the root of the nose to the occipital protuberance, is 11.75 centimetres. =2. The biparietal= (B.I.P.) is the longest transverse diameter, being the distance between the parietal protuberances, and measures 9.25 centimetres. =3. The bi-temporal= (B.T.) is the greatest distance between the temporal bones and measures 8 centimetres. =4. The occipito-mental= (O.M.) is the greatest distance from the lower margin of the chin to a point on the posterior extremity of the sagittal suture, and measures 13.5 centimetres. =5. The sub-occipito bregmatic= (S.O.B.) is measured from the under surface of the occiput, where it joins the neck, to the centre of the anterior fontanelle, a distance of 9.5 centimetres. The greatest circumference of the fetal head is at the plane of the occipito-mental and biparietal diameters and measures 38 centimetres. The smallest circumference is at the plane of the sub-occipito-bregmatic and biparietal diameters, and measures 28 centimetres. These figures, however, like all of those which it is possible to give, simply represent averages taken from a large number of cases. Individual variations will be found among normal babies, for boys’ heads, for example, are usually larger than girls’ while the head of the first child is likely to be smaller than the heads of those born subsequently. CHAPTER V SIGNS, SYMPTOMS, AND PHYSIOLOGY OF PREGNANCY