Uterine stimulants and relaxants

Uterine stimulants and relaxants, as their names suggest, are medications used to induce or inhibit contractions of the uterus.

Uterine stimulants are also called uterotonics, because they increase the tone of the muscles of the uterus.

They’re mainly used to induce or facilitate labor, to reduce postpartum hemorrhage, and to induce abortion.

Uterine relaxants are also called tocolytics, where ‘toco-’ refers to childbirth, and ‘-lytic’ means to terminate.

They’re used to stop premature labor inorder to give time for the fetal lungs to mature.

So, the uterus is a super important organ in the female body, because not only does it allow the development of the baby during pregnancy, but it also generates the force necessary to push out that tiny human during labor.

This force results from the muscle contractions that take place in the middle layer of the uterus, the myometrium.

The regulation of these muscle contractions happens at the cellular level, and it all comes down to the activity of myosin light-chain kinase by calcium ions.

This enzyme phosphorylates myosin, inducing muscle contraction.

On the other hand, this enzyme can be turned off via phosphorylation by protein kinase A, which causes the muscles to relax.

Now, during labor, uterine contractions become stronger and more frequent.

Normally, this happens at some point when the fetus is considered full term, which means all their organs are functional and it’s able to live outside mommy’s womb. This typically happens between 37 and 42 weeks’ gestation.

Sometimes labor doesn’t start even though it’s been more than 42 weeks. In these cases, it should be induced by the administration of uterine stimulants which include oxytocin, ergonovine, and prostaglandins.

In other situations the baby, the mother, or both, are at risk if the pregnancy continued.

These include fetal growth restriction, isoimmunization, premature rupture of the membranes, pre-eclampsia, or eclampsia.