Fetal development

In the seventh and eighth weeks of development, the head becomes more erect, and the previously curved trunk becomes straighter. The heart and liver, which earlier dominated the shape of the ventral body, yield to a more evenly rounded chest-abdomen region. The tail, which at an earlier time was one-fifth of the embryo’s length, becomes inconspicuous both through actual regression and through concealment by the growing buttocks. The face rapidly acquires a fairly human appearance; eyes, ears, and jaws are prominent. The eyes, previously located on the sides of the head, become directed forward. The nose lacks a bridge and so is of the “pug” type, with the nostrils directed forward instead of downward. A mandibular branch of each Y-shaped branchial arch combines with its mate to form the lower jaw. The maxillary branch on each side joins an elevation on the medial (inner) side of the corresponding nostril to produce the more-complicated upper jaw. Branchial arches, other than those forming the jaws and external ears, are effaced through incorporation into an emerging recognizable neck. Limbs become jointed, and the earlier hand plates and foot plates differentiate terminal digits. Primitive external genitalia appear, but in a nondistinctive, sexless condition.

Almost all of the internal organs are well laid down at the end of eight weeks, when the embryo is little more than 25 mm (1 inch) long. The characteristic external features are established, and subsequent growth merely modifies existing proportions without adding new structure. Similarly, the chief changes undergone by internal organs and parts are those of growth and tissue specialization. At eight weeks the neuromuscular mechanism attains a degree of perfection that permits some response to delicate stimulation.

During the third month the young fetus clearly resembles a human being, although the head is disproportionately large. The previous protrusion of much of the intestine into the umbilical cord is reduced through the return of its loops into the abdomen. The ears rise to eye level and the eyelids fuse shut. Nails begin forming; ossification (bone-forming) centres appear in most of the future bones; and the sex of external genitalia becomes recognizable. (In this paragraph, and in the next two, the months are lunar months, of 28 days.)

At four months individual differences between the faces of fetuses become distinguishable. The face is broad but the eyes are now less widely separated. The umbilical cord attaches higher on the abdominal wall; this location is above an expanding region between the cord and the pubis (front bones of the pelvis) that scarcely existed previously.

At five months downy hairs (lanugo) cover the body, and some head hairs appear. The skin is less transparent. Fetal movements (“quickening”) are felt by the mother. At six months eyebrows and eyelashes are clearly present. The body is lean, but its proportions have improved. The skin is wrinkled. Its reddish, wrinkled skin is smeared with a greasy substance (vernix caseosa). The eyelids reopen. At eight months fat is depositing beneath the skin. The testes begin to invade the scrotum. At nine months the dull redness of the skin fades and wrinkles smooth out. The body and limbs become better-rounded.

At full term (38 weeks) the body is plump and proportions are improved, although the head is large and the lower limbs are still slightly shorter than the upper limbs. The skin has lost its coat of lanugo hair, but it is still smeared with vernix caseosa. Nails project beyond the finger tips and to the tips of the toes. The umbilical cord now attaches to the centre of the abdomen. The testes of males are usually in the scrotum; the greater lips of the female external genitalia, which previously gaped, are now in contact. Cranial bones meet except at some angular junctions, or “soft spots.”

The average time of delivery is 280 days from the beginning of the last menstrual period, whereas the duration of pregnancy (age of the baby) is about 266 days (38 weeks). Pregnancy may extend to 300 days or even more, in which case the baby tends to be heavier. Premature babies born under 27 weeks of age are less likely to survive, even when treated in a neonatal unit, whereas those more than 30 weeks old usually do survive.

The average size and weight of the baby from two to nine months during prenatal development are shown in the table.