Placenta Previa

December 14, 2007

One of the most famous causes of bleeding in pregnancy, placenta previa is the positioning of the placenta in a way which blocks the cervix (the way out!). Since the placenta is such a vascular organ, any thinning of the cervix--resulting in a sliding of its base of attachment--can lead to separation.  That's what the bleeding is all about (See below). But this is more of a second trimester problem, because the cervix is so thick in the first trimester that even with a placenta previa things are usually quiet.

 

Placenta previa in the 1st Trimester

Also, since the uterus isn't that big yet in the first 12 weeks and there's limited room to fit the growing baby and the placenta, then it's not uncommon for the placenta to encroach on the area near or on the cervix, a position which will seem to "migrate" away as the uterus grows. So placenta previa is not the crisis in the first trimester that a persistent placenta previa is in the second and is not likely to be a cause of first trimester bleeding.

 

Placenta previa in the 2nd Trimester

I'm in my second trimester and I have a partial placenta previa. Will this problem go away? Will I need a C-section?

The placenta is the vascular part of the pregnancy that is adhered to the inside of the uterus (womb), this contact allowing nutrients and oxygen to pass through the maternal side to the fetal side, then on through the umbilical arteries to the baby. (See FETAL CIRCULATION.)

Not only is it important for this structure to remain adhered for the purpose of supplying the baby, but it is equally important that it not separate before the baby delivers, which would drain much of the baby's blood as well as create a hemorrhagic emergency for the mother (this separation called placental abruption). An important consideration is where the placenta attaches. If it's low in the uterus, there are two problems.

  • First of all, if it covers the way out for the baby (the "os" or cervix), it effectively creates a road block for the baby, guaranteeing disaster should labor and delivery proceed.
  • Secondly, the attachment down low is on thinner tissue of the uterus than the thicker, muscular layer higher up. Since the attachment is very vascular, after delivery when it separates the lower uterine lining doesn't have enough muscle to contract and pinch off the bleeding openings that are left on the maternal side. This hemorrhage can be life threatening and could even result in an emergency hysterectomy.

When the placenta covers the entire cervix, it is called a "total" previa. When only partially impinging on the area it is called a "partial" previa. Thankfully, total previas are rare, and most previas (previae) only encroach upon the edge of the cervical os.

Your question brings up another point. In early pregnancy, partial previas are common, because there just isn't a lot of surface area to the inside of the uterus, so any structure occupying the real estate there can commonly be positioned as a partial previa, or more likely, a "low-lying" placenta. (See above.)  As the uterus grows, the upper part of the uterus enlarges faster than the lower uterine segment, so a placenta lying over both areas will tend to grow "away" from the cervical os. We call this placental "migration," but this is a misnomer. The placenta doesn't actually move, but the tissue upon which it is embedded expands and it only appears to move up and away from the cervix. The resulting more safely positioned placenta is the same, though, no matter what the method.

When a low-lying placenta is seen in early or mid pregnancy, chances are that it will be well out of the way by the time of the third trimester, essentially making it a non-issue. If a placenta is low-lying, even at the edge of the cervix, one can still deliver vaginally, the baby's head pressing against any part of the placenta that might want to bleed. (Although you can imagine the heightened sense of vigilance needed in such a labor.) When the previa is total, C-section is mandatory.

 

 

Placental Abruption

The biggest risk to a previa is abruption (separation of the placenta before delivery). The mechanical jostling from the baby and the thinning of the attached lower uterine segment cause this complication.

(Abruption can also happen unrelated to previa, as in cocaine or cigarette use, diabetes, multiple gestation, hypertension, previous history of abruption, and having had many babies

Another consideration is microscopic bleeding from a previa which may consume all of your clotting factors in a very sneaky way, such that when really obvious bleeding begins, you don't have any clotting ability, adding to the hemorrhage problem. I know all of these things sound terrifying, but it's actually pretty rare, and most low-lying placentas never cause a problem.

So in answer to your question, you probably have a placenta that will "migrate" and therefore won't need a C-section. However, serial and frequent ultrasounds are recommended until the placenta is out of harm's way, usually by about 28 weeks. Until then, sexual intercourse is not recommended because even a harmless cervicitis bleeding episode will be misinterpreted as the big bad placenta and force your doctor to overreact to the situation.

 

 

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