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10/06/2004

Placenta previa primer

Introduction

Plenty of you have asked me, "Julie, what's placenta previa?"

Okay, no one has asked. Perhaps you already know. Perhaps you just couldn't care less. Or perhaps you have recently discovered an exciting new research tool called the Internet, much as I have. But since I cannot bear to keep this vast wealth of knowledge all to myself, I will explain.

Placenta previa describes the position of the placenta in relation to the cervix. In normal pregnancies, the placenta clings to the fundus, the upper part of the uterus. When instead it implants lower, closer to the cervix, you've got yourself a previa (fig. 1).

Placenta previa
Figure 1. You are so screwed.

There are three types: marginal, in which the placenta is located near the cervical opening; partial, in which the placenta partially covers the cervix; and total or complete, in which the cervix is entirely covered.


Incidence

True placenta previa persists in 1 in 200 live births, and 1 in 1,500 first-time mothers. Now, keeping my earlier lucky streak in mind, can any of the mathematicians among us crunch the numbers and figure out whether I should be buying a Powerball ticket?

Look, you can't win if you don't play.


Diagnosis

Low-lying placenta is often detected early in pregnancy via ultrasound. As the pregnancy progresses, however, it is common for the placenta to migrate (a misleading term, since the placenta does not actually fly south for the winter). With the growth of the uterus, the placenta is pulled safely away from the cervix in the vast majority of these cases. This suggests that early diagnosis is not an especially useful tool for predicting later complications.

The later the diagnosis of placenta previa, and the more complete the coverage of the cervical os, the more likely it is to persist until delivery. When a complete placenta previa is identified after 20 weeks or so, it is unlikely to resolve.

If placenta previa is undetected by ultrasound, it often remains undiagnosed until it manifests itself as bright red bleeding late in the pregnancy.


Causes

The causes of placenta previa are unknown, though some risk factors have been identified (fig. 2).

Risk factor Does it apply?
Smoking No
6 or more births Ahahahahahahahaha. Uh, no
Previous C-sections No
Cocaine use Not, um, [cough] in the last 10 years. [Clearing throat.]
Multiple pregnancy No
Previous uterine insult, including D&Cs Yes, but they totally weren't fun so they shouldn't count.
IVF Shit shit shit fuck piss
Figure 2. Totally screwed.

Symptoms

The hallmark of placenta previa is bright red bleeding in the second or third trimester. Only about 10% of women with placenta previa reach term without bleeding. The bleeding is generally caused by changes in the uterus and cervix as the body prepares for delivery, although it can also be incited by intercourse or vaginal examination.

The average gestational age at the start of bleeding is 32 weeks; with complete previa the onset of bleeding tends to be earlier. (The relationship between the onset of bleeding and neonatal complications is inversely proportional: The earlier the mother bleeds, the greater the risk of premature birth.)

The initial bleed is usually minor and tends to stop on its own. It is almost invariably followed by a later bleed of greater severity. The bleeding is usually unaccompanied by pain, although one in five women will experience symptoms of premature labor such as contractions.

Nearly 100% of women diagnosed with placenta previa will, at one time or another, freak the fuck right out.


Effects

The danger to the mother is minimal if she's getting proper care. The main risk is from hemorrhage, which can generally be treated with transfusions and fluid replacement as necessary. (And, no, "fluid replacement" does not mean "a succession of frosty-cold shots of Grey Goose." Try "large-bore IV," jackass.)

The risk to the baby is greater. The mortality rate for previa babies seems to hover somewhere near 10%, triple the neonatal mortality rate overall. 60% of these deaths occur from conditions related to premature birth. Premature delivery will occur in about two-thirds of previa cases.

Aside from complications of prematurity, previa babies also seem to experience a higher incidence of growth restriction and congenital physical anomalies.


Treatment

There is no treatment that can move the placenta out of harm's way, but feel free to think really hard about, you know, the slow inexorable progress of glaciers, the geological wonder of plate tectonics, et cetera.

In the absence of bleeding, expectant management is indicated. Patients may be advised to restrict their activities, eliminating exercise, lifting, strenuous movments, and especially sexual intercourse. Pelvic and rectal examinations are strictly contraindicated. Because the placenta could easily get nicked by careless noodling, nothing should be introduced into the vagina (fig. 3).

Itemsvagina_1
Figure 3. Wait, did I say you're screwed? No. You'd better not be.

If the patient remains asymptomatic, a scheduled C-section is performed once amniocentesis has established adequate fetal lung maturity.

If there's bleeding, on its first occurrence mother and baby will be evaluated. Upon cessation of bleeding, if their condition is stable, and if the mother can be counted on to be psychotically compliant, and if she lives close enough to a hospital to crawl there on her own in the dead of a midwinter night, tangled in her blood-soaked sweatpants and Steely Dan T-shirt, bed rest at home may be prescribed.

Upon subsequent and more serious bleeding episodes...

  • If the baby shows no signs of distress and the pregnancy is significantly pre-term, the mother may be given blood transfusions and medication to prevent premature labor. She may also be treated with steroids to hasten the maturation of the baby's lungs. Hospitalization until delivery is generally required. (Medical personnel are advised to be aware of the likelihood of a recurrence of the earlier freak-the-fuck-right-out, as later flare-ups are invariably more severe.)

  • If the pregnancy is close to term, and amniocentesis shows that the baby's lungs have sufficiently matured, a C-section may be performed. The baby will probably be fine.

  • If the pregnancy is not close to term, but the mother and/or baby are in significant distress, a C-section may be performed. The baby might not be fine.


Conclusion

Now aren't you glad you asked?


About the Author

Julie, who knows how to use Google, is currently working on a book proposal (working title: So You Have Placenta Previa, You Poor Sad Bastard: A Dry, Factual, and Not-At-All-Alarmist Guide). She is available upon request for thought-provoking lectures and reassuring patient consultations.

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