How reliable are lung maturity tests?

I am 34 weeks pregnant with my third child. Because of some complications, the doctor recommended an early delivery. Because I was on birth control pills when I got pregnant, we are unsure of my conception date. Due to this, my obstetrician has recommended an amniocentesis at 37 weeks to determine if the baby's lungs are mature. Can you tell me about lung maturity tests?


There are a number of choices that can be made in this situation. When pregnancy dating is accurate, the likelihood of a baby having difficulty breathing at 37 weeks is small (one to two percent or less). Those that do have difficulty breathing can usually be helped with some extra oxygen; it is rarely necessary for the baby to go on a respirator. When we look at babies born at term with spontaneous labor, we still see about one percent with some breathing problems, so waiting until normal labor is not completely risk free anyway.

Still, the American College of Obstetricians and Gynecologists suggests that if delivery is to be induced, and the patient is less than 39 weeks or may not have accurate dating, an amniocentesis is recommended. The amnio tests for a variety of chemicals to see if they are high enough to indicate a good chance of breathing without problems. Even then, there is a low risk of breathing problems -- somewhere in the neighborhood of one half to one percent.

The risks to an amnio at 37 weeks are minimal, if done by an experienced person. The usual risks, such as bleeding, premature rupture of membrane and infection, which cause loss of the pregnancy in earlier weeks (like before 20 weeks for the genetic type of amnios), just cause a reason for delivery at 37 weeks. Even the rare bleeding problem which might cause fetal distress can be handled with a Cesarean section (which is why we use the fetal monitor on our amnio patients for about an hour after the amnio). So, while no procedure is risk free, there are very small risks associated with an amnio at that time (using the ultrasound to guide the needle, of course).

So you have to decide between going with the delivery, knowing there is only a small chance of the baby having lung problems or having an amnio to confirm the maturity of the baby's lungs more certainly than just with the statistics for a baby that size. I would factor in how certain your dates are. If you had an ultrasound in the first 20 to 24 weeks of pregnancy, your dates should be fairly accurate, even if you were on the pill. If not, then there is a fair error rate which could put you as early as 34 weeks, where the chances of the baby having some trouble breathing is higher.

You also have to factor in whether or not your doctor will deliver you early if you don't have an amnio. If not, what are the risks to the baby from waiting? The other thing to know about amnios is the results will tell you if there is a good likelihood of the baby's lungs being able to breathe air or if we don't know. The results that are less than our normal cutoff for mature lungs just tell us there is a higher chance for immature lungs. But, at 37 weeks, the majority of babies with a test result that says they may have immature lungs are actually ready to breathe air; we just can't tell you for sure.

In any case, the outcome is likely to be good with or without the amnio. Talk to your care provider and decide which way you would rather go.

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