When Placenta Previa Occurs
There are four types or degrees of placenta previa: lateral, in which the lower margin of the placenta dips into the lower uterine segment and the major portion of the placenta is normally attached to the upper uterine segment; marginal, in which the placenta reaches the internal os when it is closed, but does not cover it; partial, in which the placenta covers the closed internal os, but does not do so when the os is fully dilated; and complete, in which the placenta covers the os, even when the cervix is fully dilated.
Placenta previa occurs in approximately 1:200 pregnancies, and it is more common in multiparous than primiparous women. Large placental size is a risk factor. Women with a history of cesarean birth have been shown to have a 3.9 percent incidence of placenta previa in a later pregnancy, possibly due to the presence of the scar on the lower uterine segment. Other risk factors include increased maternal age, parity independent of age, and cigarette smoking.
Some women with placenta previa experience a number of small episodes of painless bleeding. In fewer than 20 percent of cases is there no warning bleeding. Severe bleeding is more usual as labor progresses or following obstetrical interference such as a vaginal examination. The main clinical feature is absence of pain. A pregnant woman who reports blood loss should be asked by her midwife to save soiled clothing so extent of blood loss can be assessed. Once the blood loss has subsided, a gentle speculum exam to exclude cervical causes of bleeding may be performed.
Although bleeding from placenta previa in its early stages is not immediately life-threatening to mother and fetus, without prompt attention the situation can quickly deteriorate. Risks to the mother include shock from hypovolemia. The prognosis for the fetus is far from good in cases where the area of placental separation is significant. Hypoxia occurs due to diminution of placenta blood flow from maternal hypotension and anemia, in addition to a reduction in the area of attachment. -Louise Silverton, The Art and Science of Midwifery
Placenta Previa at Term
If no serious hemorrhage has made it imperative to act earlier, at about the thirty-eighth week the mother with placenta previa is examined in hospital. A careful one-finger manual exam is done to determine if the placenta is encroaching upon the lower uterine segment, in which case the practitioner will rupture the membranes and normal labor will ensue; or if the placenta reaches the margin of the cervix either posteriorly or anteriorly or covers the os, in which case a cesarean will be performed.
Postpartum hemorrhage may complicate the third stage of labor since there are few oblique muscle fibers to control bleeding from the placental site in the lower uterine segment.
In mothers who have had previous cesarean sections, placenta previa accreta may occur when the placenta is morbidly adherent to the previous uterine scar. Serious hemorrhage may occur when attempts are made to separate the placenta. -Betty R. Sweet, Mayes' Midwifery, Bailliere Tindall, 1988.
Placenta Previa: Predisposing Factors
1. Women over 35 are three times more likely to have a placenta previa.
2. A large, thin placenta. These are more likely to cover the cervix simply because they take up more of the uterine surface.
3. Multiparity. Theoretically, 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.
4. 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.
5. Endometritis following a previous pregnancy.
6. Uterine scars. In one study, the rate of previa in an unscarred uterus was 0.26 percent, 0.65 percent after one cesarean, 1.8 percent after two, 3 percent after three, and 10 percent after four or more, a 38-fold increase.
7. Repeated pregnancies with short intervals in between.
8. A history of previa. Women with a history of previa are twelve times more likely to have a recurrence.
9. Multiple gestation, due to the large placental implantation site(s).
10. Any disturbance of the healthy formation of the uterine lining.
-Anne Frye, Holistic Midwifery Volume 1, Labrys Press, 1995.
Reprinted from Midwifery Today E-News (Vol 1 Issue 10, Mar. 5, 1999)
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