After 28 weeks of gestation, the placenta attaches to the lower part of the uterus, even when the lower edge of the placenta reaches or covers the inner cervical opening and its position is lower than the previa, which is called placenta previa.
Placenta previa is a serious complication in late pregnancy and the most common cause of vaginal bleeding in late pregnancy. Its incidence is 0.5% in foreign countries and 0.24%-1.57% in China.
[Classification].
According to the relationship between the lower edge of the placenta and the inner cervical opening, placenta praevia is divided into 3 categories.
1, complete placenta previa, also known as central placenta previa, the placenta tissue completely covers the endocervix.
2.Partial placental previa, the placental tissue partially covers the endocervix.
3, Marginal placental previa: The placenta is attached to the lower part of the uterus and the edge of the placenta reaches the endocervix without covering the endocervix.
Low placenta: The placenta is located in the lower segment of the uterus, and the edge of the placenta is extremely close to but does not reach the endocervix.
Placenta previa status: those with placenta previa detected by B-mode ultrasonography in the middle of pregnancy (<28 weeks).
The relationship between the lower edge of the placenta and the endocervical opening may change due to the disappearance of the cervical canal and dilatation of the uterine opening.
The type of placenta praevia can change depending on the period of diagnosis. For example, complete placenta praevia before delivery and partial placenta praevia after delivery due to dilatation of the uterine orifice.
At present, the classification is determined clinically according to the last examination result before treatment.
[Etiology].
The cause of placenta praevia is not known, but women with advanced maternal age (>35 years), menstrual and multiple births, and women who smoke or use drugs are at high risk. The etiology may be related to the following factors.
1. Endometrial lesion or injury
(1) History of multiple curettage, delivery and uterine surgery are high-risk factors for placenta praevia. These conditions can damage the endometrium and cause endometritis or atrophic lesions, and when conceived again, the uterine metaplasia vascular formation is poor and the placenta blood supply is insufficient, stimulating the placenta to increase in size and extend to the lower part of the uterus.
(2) The scar of the previous cesarean section can prevent the placenta from migrating upward in late pregnancy, increasing the possibility of placenta praevia. According to statistics, 85%-95% of pregnant women with placenta praevia are menstruating mothers.
2.Placental abnormalities
(1) The incidence of placenta praevia is 1 times higher in twin pregnancies than in singleton pregnancies.
(2) normal position of the placenta and the parietal placenta is located in the lower part of the uterus near the endocervix.
(3) Placenta praevia can occur when the membranous placenta is large and thin and extends to the lower uterine segment.
3. Delayed development of the trophoblast of the fertilized egg
After the fertilized egg arrives in the uterine cavity, the trophoblast layer has not yet developed to the stage where it can implant, and continues to travel downstream to the lower uterine segment, where it implants and develops into placenta praevia.
[Clinical manifestations
1. Symptoms.
The typical symptom of placenta praevia is the occurrence of unprovoked, painless recurrent vaginal bleeding in late pregnancy or at the time of labor.
(1) The lower part of the uterus gradually stretches in late pregnancy, pulling the inner cervical opening and shortening the cervical canal; regular contractions after labor make the cervical canal disappear and become part of the soft birth canal. The external cervical opening dilates, so that the placenta previa attached to the lower uterine segment and the endocervical opening cannot stretch accordingly and separate from its attachment, and the blood sinus ruptures and bleeds.
(2) There is no obvious cause before the placenta praevia bleeding, and the initial bleeding is usually small, and the bleeding stops naturally after the blood coagulates at the detachment; there are also cases of fatal hemorrhage leading to shock at the first time.
(3) Due to the continuous stretching of the lower uterine segment, placenta praevia bleeding often occurs repeatedly and the bleeding volume becomes more and more.
(4) The early and late occurrence of vaginal bleeding, the number of recurrent occurrences and the amount of bleeding are related to the type of placenta praevia.
(1) The first bleeding of complete placenta praevia is early, mostly around 28 weeks of pregnancy, which is called “alert bleeding”.
(2) Marginal placenta praevia bleeding mostly occurs in late pregnancy or after delivery, and the amount of bleeding is less.
In partial placenta praevia, the time of first bleeding, the amount of bleeding and the number of repeated bleeding are in between.
2.Signs, the general condition of the patient is related to the amount of bleeding, a large amount of bleeding
(1) Presenting shock manifestations such as pale face, increased and weak pulse rate, and decreased blood pressure.
(2) Obstetric examination: uterus is soft, no pressure pain, size is consistent with the number of weeks of gestation. The examination at the time of labor shows that the contractions are paroxysmal and the uterus is completely relaxed during the interval.
When the anterior placenta is attached to the anterior wall of the uterus, a placental murmur can be heard above the pubic symphysis. Repeated bleeding or excessive bleeding at one time may cause intrauterine hypoxia and in severe cases fetal death in the uterus. The fetal previa is high floating (due to the placenta occupying the lower part of the uterus, which affects the entry of the fetal previa into the pelvis). It is easy to complicate the abnormal fetal position.
Diagnosis
1. Medical history and clinical manifestations
For patients with previous history of multiple scrapings and deliveries, history of uterine surgery, history of smoking or abuse of narcotic drugs, or history of advanced pregnancy or twin pregnancies, with the above mentioned symptoms and signs, preliminary judgment can be made on the type of placenta praevia.
2.Auxiliary examination
(1) B-type ultrasonography can clearly show the position of uterine wall, placenta, previa and cervix, and determine the type of placenta praevia according to the relationship between the lower edge of placenta and inner cervical opening.
(2) When diagnosing placenta praevia with B-mode ultrasound, the number of weeks of pregnancy must be noted.
(1) The placenta occupies half of the uterine wall in the second trimester, so there is more chance for the placenta to be close to or cover the endocervical opening.
The formation and extension of the lower uterine segment increases the distance between the endocervical opening and the edge of the placenta, so the placenta that seems to be in the lower uterine segment can change to a normal position with the upward movement of the uterine body.
(3) Therefore, many scholars believe that if placenta previa is detected by B-mode ultrasonography in the middle of pregnancy, it should not be diagnosed as placenta praevia, but should be called placenta previa.
(3) Vaginal B-mode ultrasound can more accurately determine the relationship between the edge of the placenta and the inner cervical opening.
3.Postpartum examination of placenta and fetal membranes
(1) For patients with antepartum hemorrhage, the postpartum period should be carefully examined for vascular rupture at the edge of the fetal surface of the placenta, which can indicate the presence of paracentral placenta.
(2) If there is old black-purple blood clot attached to the maternal side of the placenta in the anterior part, or the distance of the fetal membrane rupture from the edge of the placenta is <7cm, then it is placenta praevia.
[Differential diagnosis
Placenta praevia should be mainly differentiated from type I placenta abruptio, umbilical cord sail attachment, ruptured anterior vessels, ruptured blood sinus at the placental margin, cervical lesions and other antepartum hemorrhage. Combining with medical history, B-type ultrasound examination and examination of placenta after delivery, it is generally not difficult to differentiate.