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Pharmacology for Preterm Labor (OB Nursing)

Nurse Mike (Mike Linares)
By SimpleNursing | Published August 13th, 2018
Published August 13th, 2018
A pregnant Asian woman sits with her hands on her belly as she visits a mixed race female doctor. The doctor is wearing blue scrubs and has her back to the camera.

Medications for preterm labor are mainly given to slow down the rate of contractions occuring inside the uterus. The primary mechanism of action of preterm labor drugs is to prevent or decrease the influx of calcium that triggers these premature contractions.

We’ll get into the different medications, their effects, dosage, and other essential information that you need to know to pass your OB nursing pharmacology exam.

Magnesium Sulfate

One of the most popular medications for preterm labor is magnesium sulfate. This drug is a favorite for nursing tests, so pay close attention to the details we’ll emphasize.

Magnesium Sulfate Mechanism of Action (The Handgun)

In our previous lectures, we’ve talked about magnesium sulfate being the handgun of an electrolyte party, calming down whatever activity that’s happening. Magnesium sulfate prevents the body from overreacting by blocking calcium’s ability to enter the cell.

A nifty trick to remember calcium’s mechanism for pregnant women is that, think of calcium as the component for hardening bones. Calcium affects the pregnancy by making the uterus hard or contracted. You want a soft and relaxed uterus for preterm labor, which is why magnesium sulfate is given.

But don’t get confused; calcium for the bone, a vitamin, is different from calcium for the uterus, which is an ion.

Magnesium Sulfate Loading Dose

Magnesium sulfate’s loading dose is 4 to 6 grams in 30 minutes.

For those who are unaware of what a loading dose is, think of it as a larger dose that quickly achieves a therapeutic range. Certain medications cannot be given to pregnant women. Those that can be given in pregnancy are administered with caution. In cases of preterm labor, a loading dose of magnesium sulfate is given to reach the desired therapeutic range.

The loading dose is helpful because we want to achieve a therapeutic range. However, at the same time, we don’t want them to be under the therapeutic range. It is a fine line with dosing and carefully given as not to create an overdose.

Again, magnesium sulfate’s loading dose is 4 to 6 grams in 30 minutes and is given intravenously. As for the maintenance, the client will be given an IV drip of 1 to 3 grams every hour.

Magnesium Sulfate Nursing Considerations

In normal pharmacokinetics, drugs are absorbed, distributed, and excreted out of the body. All drugs that are absorbed, distributed, and metabolized are excreted through the kidneys. And magnesium sulfate is one of those drugs that can severely affect the kidneys. 

Therefore, checking the BUN and creatinine is important before giving magnesium sulfate. Other drugs that affect the kidneys greatly are antibiotics like vancomycin and gentamicin.

Checking the toxicity levels of your kidneys is important. It must be done every four hours when a pregnant client takes magnesium sulfate to ensure the drug is not damaging the kidneys and is within the healthy therapeutic range.

Magnesium Sulfate Antidote

If you go beyond the desired therapeutic range, the pregnant client will experience an overdose. If a client overdoses, it means that the drugs will affect the uterus, heart, and lungs.

Magnesium sulfate blocks calcium – therefore, the antidote for an overdose is calcium, called calcium gluconate. To reverse the effect of an overdose, an antidote is needed, and this is not only in the case of magnesium sulfate but for other medications as well. Narcan is the antidote for narcotics or opioid overdose.


Another medication given to stop premature contractions is Terbutaline. Terbutaline is a beta-adrenergic receptor agonist.

As a quick recap, beta-1 is for the heart, and beta-2 is for the lungs. To help increase heart rate and respiratory rate, beta-adrenergic agonist drugs are given. By giving Terbutaline, the lungs will expand, and the heart will relax, resulting in affected early contractions.

Terbutaline Mechanism of Action

Terbutaline belongs to a class of drugs known as beta-2 adrenergic agonists. It stimulates beta-2 adrenergic receptors in the smooth muscles of the uterus. This stimulation leads to the relaxation of the uterine muscles, which helps to suppress contractions and delay the onset of labor. 

By effectively inhibiting premature contractions, terbutaline aims to provide sufficient time for interventions that promote fetal lung maturation and minimize the risks associated with preterm birth.

Terbutaline Nursing Considerations

Assess your client and watch for signs of increased respiratory and heart rate because this medication can potentially create tachycardia. The heart rate can go as high as 110 bpm.

Beta-adrenergic agonists also increase glucose in the bloodstream; therefore, be careful with this drug when dealing with clients with gestational diabetes. Usually, Terbutaline is withheld if the client has diabetes.

Terbutaline Loading Dose

The loading dose for Terbutaline is 2.5 mg and is given subcutaneously. To maintain the therapeutic range, 2.5 mg to 5 mg of Terbutaline is given orally every four hours.

Remember, these preterm labor drugs will help decrease contractions, helping the uterus relax and not get over-excited. Always watch out for huge indications of overdose for both drugs to not risk your client’s life.

Enhance Your Pharmacology Knowledge and Ace Your Nursing Exams

As nursing students, you know that studying pharmacology can be challenging. SimpleNursing offers comprehensive resources specifically tailored to nursing students like you. With access to engaging lessons, interactive flashcards, and practice quizzes, you can confidently navigate the complex world of pharmacology. 

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