USMLE® Step 1 style questions USMLE
A 30-year-old woman, gravida 2 para 1, comes to the labor and delivery triage at 37 weeks gestation due to mild vaginal bleeding for the past 2 hours. She has not had any pain but has had occasional, non-painful contractions. She did not receive prenatal care but states that she took prenatal vitamins throughout the pregnancy. The patient’s firstborn was delivered via cesarean section. Temperature is 37.0°C (98.6°F), pulse is 100/min, and blood pressure is 115/80 mmHg. On physical examination, the patient appears comfortable. The uterus is soft and nontender. Streaks of blood are seen in the vaginal vault with no active bleeding. The fetus is in a vertex position. The fetal heart rate is 150 bpm. Which of the following is the most likely diagnosis?
Placenta previa exam links
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
Placenta previa means “placenta first,” because the placenta is the first thing within the uterine cavity.
In this condition, the placenta implants in the lower uterus, close to or even covering the uterine opening, called the internal cervical os, and it can therefore easily bleed, which usually happens after 20 weeks of gestation.
Normally the placenta implants in the upper uterus, and it's unclear why it implants in the lower uterus.
One hypothesis is that the placenta implants lower down when the endometrium in the upper uterus is not well vascularized.
In fact, endometrial damage from things like a previous cesarean section, an abortion (which could be induced or spontaneous), uterine surgery, and multiparity or multiple pregnancies can decrease vascularization and increase the risk of placenta previa.
In other cases, risk factors for placenta previa include having multiple placentas or a placenta with a larger than normal surface area, which can both happen with twins or triplets, as well as maternal age of 35 years or more, intrauterine fibroids, and maternal smoking.
Placenta previa is classified by how close the placenta is to the cervical os, it can be complete where the placenta completely covers the cervical os; partial where the placenta partially covers the cervical os; or marginal where the edge of the placenta extends to within 2 cm of the cervical os.
As the pregnancy progresses, the lower uterine segment grows, and if the placenta’s in the lower uterus, this growth disrupts the placental blood vessels, which can cause bleeding.
This usually a sudden onset of painless bright red bleeding that typically happens after 20 weeks gestation.
The amount of bleeding can vary, and it can be intermittent or continuous, sometimes increasing during labor because of uterine contractions and cervical dilation.
Placenta previa is a pregnancy complication in which the placenta implants in the lower uterus and partially or fully covers the internal cervical os, making vaginal delivery difficult or impossible. It can cause heavy vaginal bleeding and a serious threat to both the mother and the fetus. Symptoms include painless vaginal bleeding in the third trimester, and some risk factors are previous placenta previa, multiple gestation, and uterine fibroids.