Main causes of bleeding in the second half of pregnancy
Bleeding during this period, especially massive bleeding, poses a colossal threat. The third trimester is considered the most critical. All causes are traditionally divided into two groups.
The most frequent obstetric pathologies include:
- Placenta praevia (placenta praevia).
- Premature detachment of a normally situated placenta (placental abruption — abruptio placentae).
- Placenta accreta (placenta accreta).
- Rupture of umbilical cord vessels in velamentous cord insertion (vasa praevia).
Rare causes include pathologies associated with changes in the cervix and vagina: ectopias, polyps, malignant neoplasms (cervical and vaginal cancer), as well as ruptures of varicose veins of the vagina.
Placenta praevia: essence of the pathology and the migration phenomenon
Placenta praevia is a condition in which the edge of the placenta is located less than 3 cm from the internal os, or the placental tissue partially or completely covers it.
An interesting clinical feature is placental migration. Before the 24th week of pregnancy, praevia is diagnosed much more frequently. However, at later stages, the frequency of this pathology decreases. This occurs due to the displacement of the placenta along with its site upward. The mechanism of migration is due to the formation and gradual stretching of the lower uterine segment.
Clinical nuance: the migration process does not always proceed asymptomatically. During the displacement of the placental site, the pregnant woman may experience minor bloody discharge from the genital tract.
Evolution of classification: from palpation to ultrasound
Before the widespread introduction of ultrasound diagnostics, variants of placenta praevia were determined exclusively during labor. The doctor performed a vaginal examination with cervical dilatation of 3–4 cm.
The historical classification included the following variants:
- Central (complete) placenta praevia (placenta praevia centralis): the internal os is totally covered by placental tissue, and fetal membranes are not detected within it.
- Lateral placenta praevia (placenta praevia lateralis): a variant of incomplete coverage. Part of the placenta is detected in the os, and rough fetal membranes are located next to it.
- Marginal placenta praevia (placenta praevia marginalis): another variant of incomplete praevia. The lower edge of the placenta is located strictly at the edge of the os, and only fetal membranes are present in the os itself.
Modern ultrasound diagnostics and degrees of severity
Today, the main and most informative diagnostic method is transvaginal sonography (TVS). It allows for accurate measurement of the distance from the edge of the placenta to the internal os. If this distance is from 3 to 5 cm, it is considered a low-lying placenta. If it is less than 3 cm, placenta praevia is diagnosed.
The modern classification is evaluated by ultrasound starting from the 20th week of pregnancy and includes 4 degrees:
- Grade I: the placenta is localized in the lower segment, but its edge does not reach the internal os.
- Grade II: the lower edge reaches the os, but does not cover it.
- Grade III: the edge covers the internal os and extends to the opposite part of the lower segment (asymmetrical arrangement on the anterior and posterior walls).
- Grade IV: the placenta is symmetrically located on the anterior and posterior walls of the uterus, completely covering the internal os.
Timely diagnosis is critically important: it allows for planned delivery of the patient before bleeding begins or, in case of incomplete praevia, delivery via the natural birth canal.
Etiology and pathogenesis of placenta praevia
Two key factors are involved in the formation of abnormal placental localization:
1. Uterine factor Associated with dystrophic changes in the endometrium, which disrupt the conditions for normal placentation. Dystrophy is caused by: high parity (history of multiple childbirths), uterine scars (after cesarean section or myomectomy), intrauterine interventions (curettage), chronic endometritis, as well as smoking and drug use.
2. Fetal factor Due to a decrease in the proteolytic properties of the zygote itself. Due to weak enzymatic activity, nidation (implantation) in the optimal upper parts of the uterus becomes impossible. The zygote descends and implants in the lower parts. Under these unfavorable conditions, the development of the chorion is disrupted — in particular, physiological atrophy of the villi in the area of the decidua capsular (decidua capsularis) does not occur.
Briefly: answers to topical questions
What are the clinical characteristics of bleeding in placenta praevia?
The leading clinical manifestation of placenta praevia is uterine bleeding; placenta praevia has no specific pathognomonic clinical signs. Clinical characteristics include the following features:
- Nature of discharge — blood flows outward through the genital tract, hematomas do not form.
- Color of blood — bright scarlet.
- Pain syndrome — absent.
- State of the uterus — painless in all parts, tone remains normal.
- Intensity — depends on the number and diameter of damaged vessels.
- Features in labor — spontaneous cessation of bleeding is impossible.
The higher the degree of placenta praevia, the earlier the bleeding begins.
What is the management tactic for a patient with massive bleeding against the background of placenta praevia in the third trimester?
In case of massive bleeding against the background of placenta praevia, emergency delivery by cesarean section is indicated. The action algorithm includes the following steps:
- Exclusion of placental abruption (abruption placentae) is a primary task, performed using external obstetric examination and ultrasound.
- Speculum examination — performed to exclude pathologies of the cervix and vagina if abruption is not confirmed.
- Vaginal examination — performed strictly with an operating room fully prepared.
Any variant of placenta praevia accompanied by bleeding during pregnancy is an absolute indication for emergency cesarean section. In case of increased bleeding, laparotomy is performed immediately.
What fetal complications does placenta praevia cause?
The condition of the fetus in placenta praevia directly depends on the volume of blood loss and the severity of maternal anemia or hemorrhagic shock. The main fetal complication is:
- Acute fetal hypoxia — develops against the background of profuse uterine bleeding.
Even insignificant but recurrent spotting can lead to the development of anemia in the mother, which also negatively affects the condition of the fetus.
The term placenta praevia is easy to remember through a literal translation from Latin: prae means "before", and via means "way". The placenta literally stands "in the way" of the fetus to the exit from the uterine cavity.
Class and exam questions
What is the difference between low-lying placenta and placenta previa?
In low-lying placenta, the edge of the placenta is at a distance of 3 to 5 cm from the internal os. If this distance is less than 3 cm or placental tissue covers the os, praevia is diagnosed.
What is placental migration and is it dangerous?
This is a natural process of upward displacement of the placenta due to stretching of the lower uterine segment. It is not dangerous, but minor bloody discharge from the genital tract may occur during migration.
Is vaginal delivery possible with placenta previa?
Delivery via the natural birth canal is possible only with incomplete placenta praevia. In other cases, timely operative delivery before the onset of bleeding is required.
What else to study on this topic
- Placental abruption
- Placenta accreta: pathogenesis and risks
- Rupture of umbilical cord vessels in velamentous insertion (vasa praevia)
- Management tactics for pregnancy with spotting during placental migration
- Ultrasound criteria for assessing the lower uterine segment
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