Preterm Labor and Tocolytics

December 14, 2007

Prevention, diagnosis, and the philosophical challenge of knowing the difference between treatment and overtreatment

Q & A: My doctor put me on medicine to stop contractions, even though my cervix hasn't begun dilating.  Is it necessary?  If so, will it succeed or fail?

< Management of preterm labor (PTL) is a serious obligation of any obstetrician.  Many times the cause of contractions before term is never found.  Also, often even the most rhythmic, persistent contractions seem to do no harm at all, resulting in unnecessary overtreatment. The problem is that we don't know who we're overtreating, and if we get a baby to term we'll never know whether we were over-reacting or not.  But then again, we'd rather have a good mystery than a bad "known."

The cervix ultimately responds to labor. In fact, that is what the real definition of labor is: A change of the cervix. I don't mean the slow transition of thinning (effacement) over the weeks before delivery, but an obvious change over a specific time of observation. For instance, if you were to notice contractions every 3 minutes and went to the hospital, if the cervix were to show no changes over an hour, then this is considered "false" labor. If there were a definite change, then it would be considered "active" labor or possibly even "latent" phase--that awkward time before active labor brings you past 3 or 4 centimeters dilation. In every hospital every day there are those who get sent home with false labor.  For sure it is a disappointment at term, but augmenting such a phase may lead to an unnecessary C-Section. When there's false labor long before term, it is reassuring that the cervix hasn't changed.

One tip is that there's usually nausea associated with the transition between false or latent phase and the active phase of labor.  When combined with the "deal-breaker" of cervical change, it is uncommon to mistakenly send someone home who needs to stay.

Telling The Difference Between Preterm and False Labor

What about preterm labor (PTL)? How can one tell the difference between false labor and preterm labor?

Usually, the same methods apply: Change of the cervix. When that happens the diagnosis is easy, and all of the medicines to stop labor are legitimate. But what about the difference between PTL and false labor BEFORE there's any change in the cervix? In other words, it might seem academic to fall back on the reassurance of no cervical change to blow off a PTL episode as false labor, but what if the cervix were about to change? What if it the diagnosis of false labor were made right before satisfying the criteria for real labor?

This is the situation that scares an obstetrician--and rightly so!  Imagine sending home a woman having mild contractions at 32 weeks (8 weeks early) as "false labor," only to have her return in rip-roaring active labor, 9 centimeters, and delivery of an immature infant imminent? I wouldn't want that to happen, and neither does your own doctor. That's why you're on medicines even though your cervix hasn't changed.

We're cowards. And if we've spun a few extra wheels unnecessarily to get a good baby, so be it. Because the truth is we really don't know the risk of false labor in pre-term situations. False labor may in fact not be so harmless like it is at term. If there's rhythmicity to the contractions preterm, we go running for the ounce of prevention. Another point of confusion is what is actually happening inside the uterus. To effectively push a baby against the cervix as a dilating wedge, there must be a net vector force in one direction--out. There can in fact be seemingly powerful contractions, but all of the vector forces are in different directions so that the net force is not organized in one direction. There's just aimless contracting going on, but the cervix won't change. But in keeping with the same cowardice, we don't know when those vector forces will finally organize into one direction.

Whether false labor or PTL occurs, any organized contraction pattern before term needs to be evaluated for the known causes.  Known causes for PTL are:

  • Infection (Amnionitis)
  • Abruption (Premature separation of the placenta)
  • Large for Gestational Age babies--or, LGA (When the uterus begins contracting against a large premature baby like it were a normally sized term baby)
  • Multiple Gestation (for the same reasons as for LGA), and, or course...

The unknown reasons.

Lately there have been three tests that have helped separate out those in danger from those not in danger of preterm labor. Fetal Fibronectin determination, retrieved from the cervix with an exam, can indicate those at risk.  Although present in early pregnancy, this substance quickly declines so that it's presence in the second or third trimester is a serious warning of PTL.  A look at the cervix by ultrasound is gaining a lot of attention in picking out risky patients, but the results have been mixed up to now.  These methods are helpful, but no guarantee, so cowardice still reigns supreme.

The most common medicine to have patients on to prevent labor is terbutaline (Brethine).  It is actually an asthma medicine which relaxes smooth muscle--both in the bronchial tree and in the uterus.  It isn't approved by the FDA for this, but since it is used univerally, it is considered the standard of care. (Yutipar is the similar asthma medicine that is FDA-approved, but it is much more expensive and passed over for the more affordable Brethine.) The reason terbutaline is used so widespread is because it can be given IV, subcutaneously (by shot), of by mouth.  This means that successful management of PTL in the hospital can be easily transitioned to management at home with pills. Once home, services like home uterine monitoring can be used to give warning of disturbing trends in uterine contractions. One such company that provides this service is Matria.

Of all the drugs used, the first line of defense in PTL is magnesium sulfate. Given IV, it has an excellent safety margin, its levels can be checked with a blood test to avert toxicity, and it's been around for generations. Unfortunately, it has no oral version that works well, so it's strictly an in-hospital therapy.

Procardia, a heart medicine, also helps relax smooth muscle in the uterus. A single pill can be time-released once a day, which is an advantage over the Brethine, which sometimes must be taken as frequently as every 3-4 hours (set the alarm clock!).  It can even be added to Brethine, the effects of both being additive in preventing contractions.

All of these agents are called tocolytics, from "toco," referring to strength of the contractions + -lytic, meaning lysis, or to "break up or make go away."  The act of treating PTL is called tocolysis.

There is currently a silent war being waged between those in academics and those in private practice over diagnostics and therapies of PTL.  A lot of the methods described above are very expensive, and in the cost-containment mind set of managed care there must be justification for spending this money to save just a few exta babies.



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