Chapter 6: The Passenger: Fetus

GENERAL CONSIDERATIONS

1. Resemblance to the adult human form may be perceptible at the end of 8 weeks and is obvious at the end of 12 weeks

2. By the end of 12 weeks and sometimes sooner, the sexual differences in the external genitalia may be recognized in abortuses

3. Growth is greatest during the sixth and seventh months of intrauterine life

4. Quickening (the perception by the pregnant woman of fetal movements in utero) occurs between the 16th and 20th weeks of pregnancy. The time of quickening is too variable to be of value in determining the expected date of confinement or when term has been reached. Active intestinal peristalsis is the most common phenomenon mistaken for quickening

5. Depending on maternal body habitus, the fetal heart is audible using a fetal Doppler by the 12th to 13th weeks of gestation

6. The fetal heart is audible using a stethoscope by the 18th or 20th week

7. The average length of the fetus at term is 50 cm

8. Within wide variations, the average boy in Canada (7 pounds, 15 oz or ~3600 g) is a little heavier at birth (based on 40 weeks gestational age) than the average girl (7 pounds, 10 oz or ~3500 g)

9. In premature babies, the circumference of the head is relatively large compared with the shoulders. This fact is of clinical relevance when contemplating preterm breech delivery. As the fetus matures, the body grows faster than the head, so that at term, the circumferences of the head and the shoulders are nearly the same

FETAL OVOIDS

In its passage through the pelvis, the fetus presents two oval parts, movable on each other at the neck. The oval of the head is longer in its anteroposterior diameter, while that of the shoulders and body is longer transversely. Thus, the two ovoids are perpendicular to each other.

FETAL HEAD

From the obstetric standpoint, the fetal head (Fig. 6-1) is the most important part of the fetus. It is the largest, the least compressible, and the most frequently presenting part of the baby. Once the head has been born, rarely is there delay or difficulty with the remainder of the body.

Base of Skull

The bones of the base of the skull are large, ossified, firmly united, and not compressible. Their function is to protect the vital centers in the brain stem.

Vault of Skull: Cranium

The cranium is made up of several bones. Important ones are the occipital bone posteriorly, the two parietal bones on the sides, and the two temporal and the two frontal bones anteriorly. The bones of the cranial vault are laid down in membrane. At birth they are thin, poorly ...