OB Nursing: Pharmacology for Preterm Labor

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.

For a wide array of nursing topics you can dig into, visit our SimpleNursing website and YouTube channel.

Learning All about Magnesium in 2 Minutes or Less Pt 2

In this two-minute piece about magnesium, there are just two things that you need to remember:

  1. Magnesium = Magnum (gun)
  2. When there is magnesium, there is less excitement

The Magnum

To easily recall what magnesium is, you can think about it as a magnum which is a type of gun. How can we put magnesium in a magnum scenario?

When you are at a party, and someone suddenly pulls out a magnum, people would refrain from moving, jumping, and having a good time. When there’s a gun, people would usually hit the deck, the music stops, and everyone’s quiet. This is also true if applied in a scenario with a bank robbery. Basically, when there’s a magnum, the excitement stops.


Hypermagnesemia is an electrolyte imbalance wherein increased levels of magnesium are present in the blood. Therefore, bringing a gun to a party which causes everything to shut down, can be considered as hypermagnesemia.

If there is hypermagnesemia, there is increased magnesium inside the body, so there will be less cell excitability.


On the other hand, if there is decreased magnesium in the blood, lower than its standard levels, that’s hypomagnesemia. Again, back to the guns at a party; if you didn’t bring your magnum to a party, a festival, or a spring break, there’s no threat so people will be partying, dancing and singing, the music’s blaring, basically, everyone’s’ having a good time. This scenario is really not a bad thing, right?

However, if you have hypomagnesemia and you shift your perspective by relating that to what’s happening inside the body, that’s when things become dangerous. If there is a lot of excitement, like for example, in the heart, due to the increased electrical stimulant, there will be the presence of a tornado (Torsades de Pointes).

Magnesium Sulfate

To pacify elevated excitement happening inside the heart, magnesium sulfate must be administered to the client. Magnesium sulfate is like bringing in the big guns, doing crowd control in a rally and bringing the outlaws to justice.

What are the leading indicators for infusing magnesium sulfate?

  1. The tornado (Torsades de Pointes) is a specific type of polymorphic ventricular tachycardia that is mainly characterized by erratic and rapid QRS complexes with a tornado-like baseline. Torsades de Pointes is seen on your EKG strip.
  2. Any form of irregular ventricular rhythms like ventricular fibrillation (V-fib) or ventricular tachycardia (V-tach).
  3. Cardioversion or in layman’s term, “Big Shock.” Here, a shock is sent through the SA node down to the AV node, then to the Purkinje fibers, and through the bundle branches to get good, clean contractions and getting the heart rhythm back on track.

A Bit of Review

Remember, when you see in your EKG strip that tornado (Torsades de Pointes), magnesium sulfate is usually given to bring down the excitability. Remember, if you have a magnum, there’s no reason to be excited about that. On the other hand, magnesium sulfate will do crowd control, bring down the excitement, and get the baseline back to normal.

So that’s your two-minute rundown of magnesium, hypo and hyper magnesium, and magnesium sulfate. Hopefully, this helped with your magnesium dilemma.

Until our next lecture!