Placenta Previa

February 12th, 2010 § 0 comments

My preceptor and I had a discussion about previa sparked by my question as to whether or not previa could be the cause of bleeding in earlier pregnancy. (She said yes. I had only remembered bleeding from previa to occur in late pregnancy.) So, like all good students, I took advantage of this learning opportunity (more like opportunity to really remember what I learned), and got out the books. Here’s what I found, with my comments in italics.

Previa occurs when the placenta implants either partially or completely over the internal os of the cervix. It occurs in 1 out of every 200 births. I didn’t realize it occurred that often! If enough of the cervical opening is obstructed, softening, effacement and dilation of the cervix will cause the placenta to separate or tear around the margins of the internal os with accompanying rupture of underlying blood vessels and bleeding.
Placenta previa can become symptomatic during the late first or second trimester, but it most commonly manifests with a “warning bleed” at 28 weeks or later, because this is when significant cervical change most commonly begins. A warning bleed is usually painless, fluid, bright red bleeding, because it occurs just at the cervix. Some clots can form if the blood pools in the vagina, such as may occur during the night. Would the earlier warning bleeds occur more commonly in multips, who tend to have earlier cervical changes during pregnancy?


Predisposing factors:
Women over 35 are three times more likely to have a placenta previa. Why? Is the uterine lining more scarred the more cycles you have—similar to the increase in risk of previa if a woman has had five or more D & Cs, which also cause trauma to the uterine lining?

A large thin placenta, simply because it takes up more of the uterine surface.

Multiparity (A theory is that the trophoblast seeks out a new area to implant during each pregnancy; if several pregnancies preceded this one, there will be less such areas in the uterine lining.) But that theory doesn’t really make sense when considered in light of the increased risk of previa with a previous uterine previa. The risk is 12 times greater!

If the lining of the upper uterine segment is deficient, the placenta may spread out over more of the uterine wall in an effort to maintain an adequate blood supply.

Endometritis following a previous pregnancy. Looks like anything that causes damage to the uterine lining increases your risk of placenta previa.

Uterine scars increase the risk of previa; with four or more cesareans there is a 10% risk of previa! The risk of previa and accreta rises from 5% chance in a unscarred uterus to 24% with 1 scar, 47% with two scars, 40% with 3, and 60% with four or more cesarean scars.


Repeated pregnancies with little time between.

A history of previa makes it 12 times more likely that a woman will have another placenta previa. That seems like a surprising jump.

You would think, as the theory stated above suggests, that the placenta would seek an unscarred and “unused”, so to speak, portion of the uterus in which to implant. However, the fact that the placenta is so much more likely to implant over the cervix again if it did so before, makes this theory seem untrue. I really wonder if the increased likelihood of a repeat placenta previa is caused because the placenta is more likely to implant where it did before (or on a scarred/damaged portion of the uterus), or, if the factors that caused the first placenta previa were still present, and were a cause of the second previa. That would be something very interesting to investigate. =)

I love interesting discussions sparked by a real-life situation, or question that lead me to interesting thoughts and discoveries.

This little tangent reminds me again—interventions lead to increased risks. Cesareans are not a risk-free procedure! I’m glad we have them when we really need them, but women should be properly informed regarding their risks—especially when it is not actually necessary (elective cesareans, for instance).

The information and statistics for this post were taken from Holistic Midwifery by Anne Frye.

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