Placenta previa is a condition that occurs during pregnancy when the placenta is abnormally placed, and partially or totally covers the cervix.
Placenta previa is an abnormal placenta that is located on the lower uterine segment that covers part or all of the opening of the birth canal (ostium uteri internum).literally means the placenta is implantation is not the proper place,namely at the top of the uterus and birth canal.the detection mode course withan ultrasound examination. Placenta previa placenta is a major cause bleeding in the third trimester.
Placenta Previa Placenta praevia (placenta previa AE) is an obstetric complication in which the placenta is inserted partially or wholly in lower uterine segment. It can sometimes occur in the later part of the first trimester, but usually during the second or third. It is a leading cause of antepartum haemorrhage (vaginal bleeding). It affects approximately 0.4-0.5% of all labours. 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. If the placenta does overlie the lower segment, as is the case with placenta praevia, it may shear off and a small section may bleed.
The term placenta previa refers to a placenta that overlies or is proximate to the internal os of the cervix. The placenta normally implants in the upper uterine segment. In placenta previa, the placenta either totally or partially lies within the lower uterine segment. Traditionally, placenta previa has been categorizedinto 4 types :
1.    Complete placenta previa, where the placenta completely covers the internal os.
2.    Partial placenta previa, where the placenta partially covers the internal os. Thus, this scenariooccurs only  when the internal os is dilated to some degree.
3.    Marginal placenta previa, which just reaches the internal os, but does not cover it.
4.    Low-lying placenta, which extends into the lower uterine segment but does not reach the internal os.

The placenta grows wherever the embryo implants itself in the uterus. If the embryo implants itself in the lower portion of the uterus, the placenta might grow over the cervix — causing placenta previa. Most cases of placenta previa are diagnosed at the time of a second trimester ultrasound examination. If the placenta is just barely reaching the cervix, the situation will resolve itself because expansion of the uterus pulls the area of placental attachment higher up in the uterus, away from the cervical opening. If the placenta is found to be all the way across the cervix, however, it is unlikely to resolve with time. Persistent types of placenta previa have been associated with:
§  Scars in the lining of the uterus
§  A large placenta, such as with a multiple pregnancy
§  An abnormally shaped uterus
While the actual cause of placenta previa is unknown, certain factors increase the risk of a woman developing the condition. These factors include:
§  having abnormalities of the uterus
§  being older in age
§  having had other babies
§  having a prior delivery by cesarean section
§  smoking cigarettes
When a pregnancy involves more than one baby (twins, triplets, etc.), the placenta will be considerably larger than for a single pregnancy. This also increases the chance of placenta previa.Placenta previa may cause a number of problems. It is thought to be responsible for about 5% of all miscarriages. It frequently causes very light bleeding (spotting) early in pregnancy. Sometime after 28 weeks of pregnancy (most pregnancies last about 40 weeks), placenta previa can cause episodes of significant bleeding.
Usually, the bleeding occurs suddenly and is bright red. The woman rarely experiences any accompanying pain, although about 10% of the time the placenta may begin separating from the uterine wall (called abruptio placentae), resulting in pain. The bleeding usually stops on its own. About 25% of such patients will go into labor within the next several days. Sometimes, placenta previa does not cause bleeding until labor has already begun.
Placenta previa puts both the mother and the fetus at high risk. The mother is at risk of severe and uncontrollable bleeding (hemorrhage), with dangerous blood loss. If the mother's bleeding is quite severe, this puts the fetus at risk of becoming oxygen deprived. The fetus' only source of oxygen is the mother's blood. The mother's blood loss, coupled with certain changes that take place in response to that blood loss, decreases the amount of blood going to the placenta, and ultimately to the fetus. Furthermore, placenta previa increases the risk of preterm labor, and the possibility that the baby will be delivered prematurely.

It is unclear why some placentas implant in the lower uterine segment rather than in the fundus.10 It does appear that uterine scarring may predispose to placental implantation in the lower segment. With the progression of pregnancy, more than 90% of theselow-lying placentas identified early in pregnancy will appear to move away from the cervix and out of the lower uterine segment. Although the term “placental migration” has been used, most authorities do not believe the placenta moves.10 Rather, it is felt that the placenta grows preferentially toward a better vascularized fundus (trophotropism), whereas the placenta overlying the less well vascularized cervix may undergo atrophy.
In some cases, this atrophy leaves .vessels running through the membranes, unsupported by placental tissue or cord (vasa previa). In cases where the atrophy is incomplete, a succenturiate lobe may develop. The apparent movement of the placenta may also be due to the development of the lower uterine segment. Contractions and cervical effacement and dilation that occur in the third trimester cause separation of the placenta, which leads to small amounts of bleeding. This bleeding may stimulate further uterine contractions, which, in turn, stimulates further placental separation and bleeding. Rarely are these initial bleeds a major problem, although they may be a reason for hospitalization. In labor, as the cervix dilates and effaces, there is usually placental separation and unavoidable bleeding.

Associated findings, In cases of suspected placenta previa, a vaginal examination is delayed until ultrasound results are available and the client is moved to the operating room for what is termed a double-set-up procedure. The operating room is needed because the examination can cause further tearing of the villi and hemorrhage, which can be fatal to the client and fetus. Common clinical manifestations include:
o   Red, painless vaginal bleeding
o   Soft, nontender abdomen; relaxes between contractions, if present.
o   FHR stable and within normal limits.
o   His usually no
o   Presentations may be abnormal
o   Recurrent bleeding
o   The presence of anemia and shock in accordance with the discharge of blood
o   The emergence of when pregnant
o   Flaver is not tense (regular) when palpation
o   Fetal heart rate
o   Placental tissue palpable in the vagina in check
o   Desrease in the head does not enter the door above the pelvis
o   Presentations may be abnormal
Laboratory and diagnostic study findings. Transabdominal ultrasonography confirms suspicion of placenta previa.
v Symptoms of placenta previa
Vaginal bleeding after 20 weeks of pregnancy is the primary symptom of placenta previa. Bleeding during pregnancy may have another cause, however, it is important to call your doctor if you experience bleeding.
The placenta normally attaches to the upper portion of the uterus which is more muscular and stronger to support the placenta. However, in placenta previa the placenta attaches to the lower portion of the uterus which is weaker, thinner, and more vascular. As you enter your second and third trimester, the cervix begins to thin and stretch in preparation for labor. As this area stretches it can cause the villi (blood vessels) to break therefore causing bleeding.
Placenta previa can lead to complications for both mother and baby. Complications that may arise include placenta abruption, hemorrhaging, preterm labor, anemia for either mother or baby.Complications after delivery,Because the risk of hemorrhaging is higher for women with placenta previa, mothers will be monitored for signs of hemorrhaging. She may be given medications to control bleeding such as pitocin and a transfusion may sometimes be necessary. Anemia may occur in mother or baby therefore hemoglobin levels will be monitored and iron supplements may be given.

E.   Diagnosis
The classic clinical presentation of placenta previa is painless bleeding in the late second trimester or early third trimester. However, some patients with placenta previa will experience painful bleeding, possibly the consequence of uterine contractions or placental separation, whereas others will experience no bleeding at
all before labor. Placenta previa may also lead to an unstable lie or malpresentation in late pregnancy.
The majority of cases of placenta previa are diagnosed during routine sonography in asymptomatic women, usually during the second trimester. Although transabdominal sonography is frequently used for placental location, this technique lacks some precision in diagnosing placenta previa.Numerous studies have demonstrated the accuracy of transvaginal sonography for the diagnosis of placenta previa, uniformly finding that transvaginal sonographyis superior to transabdominal sonography for this indication .
False-positive and –negative rates for the diagnosis of placenta previa using transabdominal sonography range from 2% to 25%.11 A study by Smith and colleagues11 of 131 women believed to have a placenta previa by transabdominal sonography found that anatomic landmarks crucial for accurate diagnosis were poorly recognized in 50% of cases. In 26% of the cases of suspected placenta
previa, the initial diagnosis was changed after transvaginal sonography because it was incorrect.
The superiority of transvaginal sonography over transabdominal sonography can be attributed to several factors:
1.    The transabdominal approach requires bladder filling, which results in approximation of the anterior and posterior walls of the lower uterine segment, with the result that a normally situated placenta may falsely appear to be a previa.
2.    Vaginal probes are closer to the region of interest, and typically of higher frequency, and therefore obtain higher resolution images than transabdominal probes.
3.    The internal cervical os and the lower placental edge frequently cannot be imaged adequately by the transabdominal approach. The position of the internal os is assumed rather than actually seen.
4.    The fetal head may obscure views of the lower placental edge when using the transabdominal approach,and a posterior placenta previa may not be adequately imaged.
The improved accuracy of transvaginal sonography over transabdominal sonography means that fewer false-positive diagnoses are made; thus, the rate of placenta previa is significantly lower when using transvaginal sonography than when using transabdominal sonography.11,13 Lauria and colleagues,13 performing routine transvaginal sonography, found an incidence of placenta previa of only 1.1% at 15–20 weeks, considerably lower than the second trimester placenta previa incidence of 15–20% reported by previous investigators using transabdominal sonography.
Numerous studies have demonstrated the safety of transvaginal sonography for the diagnosis of placenta previa. Importantly, this imaging technique does not lead to an increase in bleeding. This is for 2 main reasons:
a.     the vaginal probe is introduced at an angle that places it against the anterior fornix and anterior lip of the cervix, unlike a digital examination, where articulation of the hand allows introduction of the examining finger through the cervix
b.     the optimal distance for visualization of the cervix is 2–3 cm away from the cervix, so  the probe is generally not advanced sufficiently to make contact with the placenta.
Nonetheless, the examination should be performed by personnel experienced in transvaginal sonography, and the transvaginal probe should always be inserted carefully, with the examiner looking at the monitor to avoid putting the probe in the cervix. Translabial sonography has been suggested as an alternative to transvaginal sonography and has been shown to be superior to transabdominal sonography for placental location.16 However, because transvaginal sonography is accurate, safe, and well tolerated, it should be the imaging modality of choice.
Several studies have demonstrated that the majority of placentas that are in the lower uterine segment in the second trimester will no longer be in the region of the cervix by the time of delivery .Persistence to term can be predicted based on whether or not the placenta overlaps the internal os in the second trimester, and to what extent.The later in pregnancy that placenta previa is diagnosed, the higher the likelihood of persistence todelivery.
Women who at 20 weeks have a low-lying placenta that does not overlie the internal os will not have a placenta previa at term and need no further sonographic examinations for placental location. However, the presence of a low-lying placenta in the second trimester is a risk factor for developing a vasa previa, and therefore, in these cases, a sonogram should be performed later in pregnancy to exclude that condition.
Midwives face bleeding placenta previa can take a stand made ​​a referral to the appropriate health facility . In patients with placenta previa referral should be equipped with :
§  Installation of infusion to offset the bleeding
§  To the extent possible escorted by officers
§  Equipped with sufficient information
§  Prepared blood donors for blood transfusions
Some other forms of aid to placenta previa , among others :
1.    Version Braxton Hicks The purpose of Braxton hicks is to hold the buttocks and placenta tamponade to stop the bleeding in order to save the mother .
Braxton hicks version usually performed on children who were dead or still alive . Given the danger of laceration of the cervix and the lower uterine segment , these maneuvers never performed again in large hospitals . However , if the patient is bleeding a lot , the child was dead and we have difficulty in obtaining blood or operating room the way Braxton hicks can be considered .
2.    pliers Willet Gauss The goal is to hold the head of the placenta tamponade . Where fetal scalp clamped with pliers Willet gauss and weighted down with weights 500 grams . This maneuver now is never done