In the earlier works on obstetrics it was taught that themight assume any position in the uterus, the number of positions being limited solely by an author’s imagination. Accurate observations gradually eliminated the more fanciful.
The positions which the child assumes in utero may be divided into two general classes—longitudinal and transverse. In the longitudinal the spinal column of the child is parallel to the spinal column of the mother; in the transverse it is at right angles to that of the mother, forming a cross with it. The former is normal, accounting for more than 99 per cent; while the transverse position is rare, occurring in less than 0.5 per cent.
These two general classifications of fetal position may be subdivided into more exact groups. We term these more specific positions presentations; this refers to the precise part of the fetus which presents over the bony birth passage, the pelvis. At term, 96 per cent of fetuses present by the head (cephalic presentation); 3.5 per cent with the buttocks (breech presentation); and in less than 0.5 per cent the child lies transversely, a shoulder presenting.
If, during labor, the head presents, it is ordinarily continuously flexed, so that the infant’s chin rests on its breastbone. When this occurs, the top of the back of the head first enters the mother’s pelvis and is the part of the fetus earliest visible as the birth takes place. This is the common variety of cephalic presentation, accounting for almost 99 per cent of the cases in which the head presents. Such babies frequently are born with a temporary swelling over the area which the hair whorl will later occupy; the swelling is caused by pressure, since it is the lead point in the birth process.
Occasionally, instead of being flexed, the head of the fetus is extended during labor, and the infant delivers with the face presenting. Under such conditions the features may appear as swollen as those of a badly mauled prizefighter, but within forty-eight hours the contusions and swellings have disappeared. If the head is only partially extended, the baby’s brow is the lead point.
A breech presentation, relatively rare at term, is common during the earlier months. Nearly 50 per cent of babies present by the breech before the seventh month, but all except a few revolve through 180 degrees to a cephalic at some time before the end of the ninth month. There are three types of breech presentation—frank, footling, and full. In a frank breech, much the commonest, the legs of the baby are flexed up over the abdomen so that the toes touch the shoulders and the two buttocks present over the pelvis. If it were not for the very loose joints of the fetus, this position would be almost unattainable and, to say the least, uncomfortable. After delivery it is not uncommon for the baby who delivered as a frank breech to keep his legs flexed at the hip and straight at the knee for several hours. Since the buttocks and genital area are the lead points in the birth of a frank breech, they are often badly swollen and discolored black and blue at delivery. This clears up rapidly without permanent damage. In a footling breech presentation one or both legs are held straight, as in the standing position, and act as the lead point in labor and delivery. The full breech is rare; in it the fetus sits cross-legged in the mother’s pelvis, like a tailor on his sewing bench.
A fetus presenting by the breech is sometimes manipulated by the doctor to a cephalic presentation late in pregnancy before labor begins; this procedure is termed external version.
Three common positions of the fetus
Once in two hundred or more pregnancies the fetus lies transversely and a shoulder, or occasionally an arm and hand, enters the pelvis. Natural delivery is impossible and either the infant must be turned in late labor, after the mother is deeply anesthetized, or a Cesarean section must be performed.
Presentation of Twins
In twins the ordinary proportion of the various presentations is greatly altered. Both children present by the head in 39 per cent of the cases, one by the head and the other by the breech in 36 per cent, both by the breech in 11 per cent, and one or both transversely in 14 per cent.
Determination of Fetal Position
The position of the fetus is usually determined by feeling or palpating the abdomen. From the thirtieth week on, the physician can usually identify the fetal head, back, and extremities without great difficulty. In doubtful cases X-rays will always reveal the exact presentation.