Re: What triggers the start of childbirth labor?

Nancy,

There are really two questions that need to be answered: Why does 
the uterus tolerate the baby inside it for so long? And, what 
initiates labor? As you may have suspected, there is a lot known 
but no definite answer. A third question is: if you wanted to 
find out, how would you do it? You cannot experiment on humans, 
adult or unborn baby, in a manner to try to answer the question. 
Most research has been done on fetal sheep and some monkeys. In 
the sheep a major factor is the progesterone level which falls 
just prior to the beginning of labor, but that doesn’t seem to be 
important in humans or monkeys.


The definition of labor is that there are coordinated 
contractions of the smooth muscle wall of the uterus and that 
there is progressive dilation of the cervix and descent of the 
presenting part. In false labor there is no progression towards 
delivery.

Prior to pregnancy the uterus is about the size of a small lemon. 
The great bulk of it is the thick smooth muscle wall. As 
pregnancy progresses the smooth muscle wall stretches until at 
term it is about 40 cm high. This is accomplished by the smooth 
muscle cells enlarging, not by adding more smooth muscle cells. 
When anything is put into a uterus it usually reacts with 
cramping to try to expel whatever is in it, blood clot, IUD or 
previous incomplete evacuation of a miscarriage or placenta. The 
baby is a product of both the mother and fathers chromosomes, and 
is therefore, genetically different from the mother.  The uterus 
is in no way committed to accept this large load, but does so 
until the onset of labor.

The onset of labor also means the cessation of pregnancy 
maintenance. Which means that the uterus is no longer going to 
tolerate what is inside of it. There is some speculation that 
chemicals produced by the maturing baby cause some of this loss 
of tolerance by the uterus. It is known that some birth defects 
such as anencephaly (basically no brain) and adrenal gland 
dysfunction cause a marked prolongation of pregnancy and delay in 
labor.

The major chemicals known to cause uterine contractions are the 
prostaglandins. These chemicals are made by the uterus, fetal 
membranes and the decidua, which is the tissue of the lining of 
the uterus in which the placenta is implanted. Women with 
menstrual cramps, which are usually due to prostaglandins, take 
ibuprofen or any of numerous other prostaglandin inhibitors. The 
level of prostaglandins in the mothers blood and in the bag of 
water increase dramatically during labor. Labor is often induced 
by Obstetricians by administering prostaglandins to the mother. 
Preterm labor can be stopped by prostaglandin inhibitors.

Some elements of the immune system also make chemicals called 
cytokines that increase during labor. These chemicals are 
referred to as interleukins. Needless to say, a lot is known 
about what happens. But still, not much is known about what the 
triggers are.

Normally, labor commences 260 days after implantation of the 
fertilized ovum, called a blastocyst, into the uterine lining. 
Since nobody knows when this happened, the definition of preterm 
is labor starting prior to 38 weeks after the first day of the 
last normal menstrual period. That normal period should have been 
followed 14 days later by ovulation and then within the next 48 
hours by fertilization Implantation occurs a week or so later. 
Provided the menses are regular and ovulatory at about 28-30 day 
cycles, the due date can easily be calculated. Otherwise it is 
sometimes difficult for the Obstetrician to determine if labor is 
premature or not.

Basically the answer to your question is “nobody knows”. That is 
why there will never be an end to science, every time a question 
is asked it leads to even more unanswered questions.