Pathophysiology
No specific cause of placenta previa has yet been found but it is hypothesized to be related to abnormal vascularisation of the endometrium caused by scarring or atrophy from previous trauma, surgery, or infection.
In the last trimester of pregnancy the isthmus of the uterus unfolds and forms the lower segment. In a normal pregnancy the placenta does not overlie it, so there is no bleeding. If the placenta does overlie the lower segment, it may shear off and a small section may bleed.
Women with placenta previa often present with painless, bright red vaginal bleeding. This bleeding often starts mildly and may increase as the area of placental separation increases. Praevia should be suspected if there is bleeding after 24 weeks of gestation. Abdominal examination usually finds the uterus non-tender and relaxed. Leopold's Maneuvers may find the fetus in an oblique or breech position or lying transverse as a result of the abnormal position of the placenta. Praevia can be confirmed with an ultrasound.[2] In parts of the world where ultrasound is unavailable, it is not uncommon to confirm the diagnosis with an examination in the surgical theatre.
The proper timing of an examination in theatre is important. If the woman is not bleeding severely she can be managed non-operatively until the 36th week. By this time the baby's chance of survival is as good as at full term.
Placenta previa is classified according to the placement of the placenta:
- Type I or low lying: The placenta encroaches the lower segment of the uterus but does not infringe on the cervical os.
- Type II or marginal: The placenta touches, but does not cover, the top of the cervix.
- Type III or partial: The placenta partially covers the top of the cervix
- Type IV or complete: The placenta completely covers the top of the cervix
Placenta previa is itself a risk factor of placenta accreta.
[edit]Risk factors
The following have been identified as risk factors for placenta praevia:
- Previous placenta previa, caesarean delivery,[3] or D&C e.g. used for incomplete or missed miscarriage, abortion, to treat or investigate heavy bleeding or other diagnostic purposes.
- Women who have had previous pregnancies, especially a large number of closely spaced pregnancies, are at higher risk.
- Women who are younger than 20 are at higher risk and women older than 30 are at increasing risk as they get older.
- Women with a large placentae from twins or erythroblastosis are at higher risk.
- Women who smoke or use cocaine may be at higher risk.
- Race is a controversial risk factor, with some studies finding that people from Asia and Africa are at higher risk and others finding no difference.
[edit]Intervention
An initial assessment to determine the status of the mother and fetus is required. Although mothers used to be treated in the hospital from the first bleeding episode until birth, it is now considered safe to treat placenta praevia on an outpatient basis if the fetus is at less than 30 weeks of gestation, and neither the mother nor the fetus are in distress.
Immediate delivery of the fetus may be indicated if the fetus is mature or if the fetus or mother are in distress. Blood volume replacement (to maintain blood pressure) and blood plasma replacement (to maintain fibrinogen levels) may be necessary.
It is controversial if vaginal delivery or a Caesarean section is the safest method of delivery. In cases of fetal distress a Caesarean section is indicated. Caesarian section is contraindicated in cases of disseminated intravascular coagulation.
A problem exists in places where a Caesarean section cannot be performed, due to the lack of a surgeon or equipment. In these cases the infant can be delivered vaginally. There are two ways of doing this with a placenta praevia:
- The baby's head can be brought down to the placental site (if necessary with Willet's forceps or a vulsellum) and a weight attached to his scalp
- A leg can be brought down and the baby's buttocks used to compress the placental site
The goal of this type of delivery is to save the mother, and both methods will often kill the baby. These methods were used for many years before Caesarean section and saved the lives of both mothers and babies with this condition.
The main risk with a vaginal delivery with a praevia is that as you are trying to bring down the head or a leg, you might separate more of the placenta and increase the bleeding.
Placenta praevia increases the risk of puerperal sepsis and postpartum haemorrhage because the lower segment to which the placenta was attached contracts less well post-delivery.
| Placenta praevia | |
|---|---|
| Classification and external resources | |
 | |
| ICD-10 | O44., P02.0 |
| ICD-9 | 641.0, 641.1 |
| MeSH | D010923 |
Your Placenta
During pregnancy, your body creates a temporary organ called the placenta. The placenta is necessary during pregnancy in order for your baby to grow and be healthy. After the birth of your baby, your doctor or midwife will also have you deliver the placenta, as it is no longer needed.
The placenta is created during the first trimester but is not fully functional until the second trimester when it takes over all the nutrient production and waste management for the baby. The placenta is both genetically and biologically composed of two equal parts from the fetus and the mother. It is connected to the uterine wall and connected to the baby through theumbilical cord. It receives nutrients and oxygen and filters out any waste from the mother’s blood before delivering the blood to the baby via the umbilical cord.Until the placenta takes over, a woman naturally produces progesterone to keep the baby viable. As soon as the placenta takes over it starts creating its own progesterone to sustain the pregnancy. After delivery of the placenta the cord is clamped near the baby and cut off permanently disconnecting the baby from the placenta. The entrance of the umbilical cord into the baby’s body later becomes the baby’s belly button.
Some cultures practice placentophagy which is the eating of the placenta. Those who do this believe that eating the placenta helps prevent postpartum depression and other complications. However, modern doctors and midwives do not believe that eating the placenta will actually help reduce the occurrence of postpartum depression.
In some pregnancies, placenta previa can occur. Placenta previa is a condition where the placenta covers at least part of the cervix, which is the opening that the baby will come through when born vaginally. This happens in about one of every 200 pregnancies. There are 3 types of placenta previa.
- CompleteComplete placenta previa is the placenta covering the entire cervix. This can cause bleeding throughout the pregnancy, growth retardation and congenital defects in baby. It will also mean that a woman will need a c-section when it comes time to deliver her baby.
- PartialPartial placenta previa is a partial covering of cervix is covered. This can be anywhere from 1/4 to 3/4 of the cervix. This can also cause bleeding and may lead to a c-section.
- MarginalMarginal placenta previa occurs when just a small portion, or just the edge of the cervix, is covered. In this case, as the pregnancy progresses, the placenta will generally move up and out of the way, allowing a clear passage for the baby to be born vaginally.
If you experience bleeding during your pregnancy, you should talk to your doctor and rule out the possibility of any bleeding being caused by placenta previa.
Placenta Previa Seen at the 20 Week Ultrasound
One of the most common problems spotted at the 20 week ultrasound is placenta previa. Placenta previa is where the placenta is nearby, or actually covering some or all of the cervix. It’s a potentially dangerous condition because if the placenta covers the cervix, it blocks baby’s way out, requiring a cesarean section to deliver the baby. And more seriously, as the cervix dilates towards the end of pregnancy, the placenta can be torn and bleed, which can be life-threatening to mom and baby.Complete or partial placenta previa, where the placenta covers at least a quarter, or even all of the cervix, occurs in around 1 in 200 pregnancies, and requires careful monitoring by your doctor.
And more common is a marginal placenta previa, where the placenta is close by, or touching the cervix, but not actually covering it.
At 20 weeks, when the placenta is relatively large compared to the size of the uterus, many women appear to have placenta previa. The ultrasound technician will note the position of the placenta, so your doctor is aware and can discuss it with you. The doctor will probably tell you to avoid intercourse, heavy lifting, and strenuous activity. Your husband will be thrilled that he has to do the vacuuming instead of sex.
Several moms I know, including me, had marginal placenta previa at 20 weeks. At my next obstetrician appointment, the doctor explained what it was. The position of the placenta is largely random, but there are risk factors to placenta previa, including previous cesarean sections, a history of placenta previa, and it’s more common in older moms. Apparently, smoking or using cocaine also increases the risk, my doctor told me, raising one eyebrow. What? I’ve never smoked and the only kind of coke I know comes in a red can with a swirl on it.
The good news is that for most women who have marginal placenta previa at 20 weeks, as your uterus grows, the placenta gets further from the cervix and you’ll likely be given the all-clear to get back to normal activities by 24-28 weeks. Your doctor will schedule another ultrasound to check the position of the placenta at that time. If the placenta is now clear of the cervix, then vaccuming, etcetera are back on.
I had another ultrasound at 25 weeks, and like 90% of moms with marginal placenta previa at 20 weeks, my placenta was in a normal position, and the rest of my pregnancy went smoothly, except for having to think of other excuses to not vacuum.
If you are pregnant, and experience any vaginal bleeding, get checked by a doctor asap. If you have been told you have, or may have placenta previa, it is important to let the doctor who examines you know, as they will need to take extra precautions when they examine you to avoid causing any damage to your placenta.
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