The Mechanism of Labor

A Labor Pain

The neophyte always wonders how she will know when labor actually begins. Can she distinguish the pains of childbirth from intestinal cramps or an ordinary backache? The throes of labor are unique for several reasons. In the first place, unlike other pains, they rise in slow crescendo, remain fortissimo for a brief period, and close in moderate diminuendo; or they are like a scale that makes a leisurely ascent to its high tone and is held aloft for a few beats before the several descending notes are sung.

Another distinctive feature is that a labor pain is always associated with a contraction of the uterus. If the woman feels her abdomen during the acme of such a pain, she notices a large, board-firm mass, which becomes a softish, indentable mass once again when the pain is over. The chief characteristic of these pains is their rhythmic nature, their recurrence at fixed intervals. As labor progresses, the interval from the beginning of one pain to the beginning of the next is gradually shortened from fifteen or twenty minutes at the onset to three or four minutes when labor is well under way. In addition, the total length of an in- dividual pain increases from less than half a minute at the start to more than a minute toward the end. Unlike most pains, labor pains have a complete remission between them, a happy respite in which the patient is comfortable. Usually labor pains occur first in the small of the back and after a few hours migrate down the flanks to center in the whole lower abdomen. Many compare them to exaggerated menstrual cramps, which are grinding and twisting in type. The pinkish vaginal discharge which ordinarily accompanies the pains of true labor is termed ‘show.’ It is blood-tinged mucus dislodged from the mouth of the womb (cervix) as the latter begins to dilate. Sometimes this normal show, which is to be distinguished from abnormal bleeding (frank, undiluted blood), anticipates the onset of labor by a day or more.

False Labor

True labor may be preceded by one or several discouraging bouts of false labor, especially in women who have already borne children.

Cause Not Known

What causes labor to begin? Why does it start approximately 280 days after the first day of the last menstrual period? These are simple and definite questions which no one as yet can answer. From the appearance and disappearance of the hard abdominal mass, we know that contractions of the uterus occur irregularly throughout pregnancy (Braxton-Hicks contractions), but suddenly, for no explicable reason, these painless contractions become regular and painful; then labor has begun. Quite commonly, particularly in early labor, the entrance of the obstetrician is followed by a temporary lull in the intensity and frequency of pains.

Time of Day

Almost everyone will attest that the stork is a creature with perverse habits, preferring nocturnal to diurnal flights. Actually, this is not true; it just seems true. Being up at three in the morning to receive a ‘bundle from heaven’ is an event, but missing lunch just an occurrence.

Furthermore, since many labors last for ten to twelve hours, they are likely either to begin or to end at night. John forgets that John, Jr., was born at 10 A.M., but he will not forget the snowy drive to the hospital at 3 A.M.

Time of Year

The United States Public Health Service publishes the birth and death statistics for this country annually, data most accurately prepared by the National Office of Vital Statistics.

The material is so carefully checked and counterchecked that the latest volume is always at least eighteen months behind the current date.

The 4,244,796 live births in 2009 were distributed monthly as follows, the number for the average months being placed at 100, making the year’s total 1200, each month’s birth figure being adjusted for the length of the month.

January 97.4.

February 99.6.

March 99.6.

April 95.3.

May 93.3.

June 98.0.

July 103.6.

August 105.1.

September 107.8.

October 102.5.

November 99.3.

December 99.8.

The four late summer and early fall months (July-October) have the greatest number of births (34.9%), which means that the greatest number of conceptions occur in late fall and early winter.

Rupture of the Membranes

In half the deliveries the bag of waters formed by the fetal membranes ruptures during the last hours of labor. Sometimes it remains intact to the very end, and the obstetrician has to speed the labor by rupturing it. The membranes are insensitive and can be punctured without anesthesia. A pointed instrument is introduced into the vagina and a tear snagged in them.

At the rupture of the membranes varying amounts of fluid immediatelv gush forth, and minor spurts may accompany each labor contraction thereafter. In one-eighth of the cases the membranes rupture before labor has begun (premature rupture of membranes), and such labors are termed dry labors. This is about twice as common in first pregnancies as in subsequent ones.

Dry labors have an undeservedly bad reputation; they are just the same as labors which occur with membranes intact. Actually there is no such phenomenon as a dry labor, since the cells lining the inside of the amniotic sac are constantly secreting fluid at the rate of a quart in two hours. If membranes are unruptured, as much fluid is absorbed as is secreted, but when membranes are ruptured a large portion of the water flows from the vagina and only the excess is absorbed.

If the membranes rupture before the onset of labor and the patient is within a few days of term, in nine instances out of ten labor will commence within twenty-four hours— more than likely in less than six hours. On the other hand, if membranes rupture prematurely and the patient is many weeks or even months from term, there may be a very long latent period before labor’s onset—sometimes thirty or forty days, or longer. It is routine to place these patients at complete bed-rest at home or in the hospital. Allowing pregnancy to progress several vital weeks may mean the difference between no baby and a baby. A recent patient ruptured her membranes at twenty-seven weeks and did not go into labor until six weeks later. As a result of the fortunate long latent period she now has a splendid son. However, unless the pregnancy is of twenty-six weeks or longer duration, the chance for a successful outcome is probably less than two or three per cent.

Doubt often exists as to whether the membranes have ruptured. Toward the end of pregnancy, pressure on the bladder by the fetal head sometimes causes the involuntary passage of urine, which may be confused with amniotic fluid. The submission of the fluid to chemical color tests and microscopic examination usually reveals its true character, however.