OB Nursing: Pharmacology for Preterm Labor

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Medications for preterm labor are mainly given to slow down the rate of contractions happening inside the uterus. The primary mechanism of action of preterm labor drugs is to prevent or decrease the influx of calcium which 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 when it comes to nursing tests so pay close attention to the details we’ll be emphasizing.

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 orders the body to stop overreacting by blocking calcium to go into 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. What you want for preterm labor is a soft and relaxed uterus which is why magnesium sulfate is given.

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

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 signing bonus of your therapeutic range. Medications are not given to pregnant women, but 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 don’t want these expecting moms to go over the therapeutic range and get an overdose and, at the same time, we don’t want them to be under the therapeutic range that the medication will no longer work for them.

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 Cautions

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 and must be done every four hours when a pregnant client is taking magnesium sulfate to make sure that 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 not only affect the uterus but the heart and lungs as well.

Magnesium sulfate blocks calcium; therefore, the antidote for an overdose is calcium which is called as 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; thus, affecting early contractions.

Terbutaline Cautions

Assess your client and watch out for signs of increased respiratory and heart rate because sometimes, 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 is diabetic.

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 are going to help the expecting mom to decrease contractions, helping the uterus to relax and not get over-excited. Always watch out for huge indications of overdose for both drugs to not put your client’s life at risk.

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