What is Placenta Previa?
Placenta previa is a pregnancy complication in which the placenta—the organ that supplies oxygen and nutrients to the baby—lies unusually low in the uterus and partially or completely covers the internal opening of the cervix (the os). Because the cervix must dilate for labor, a low‑lying placenta can cause severe vaginal bleeding during the second or third trimester and often necessitates delivery by Cesarean section.
The condition is named from the Latin previa meaning “lying in front.” It occurs in about 0.5 % of pregnancies, making it relatively uncommon but clinically important because bleeding can be life‑threatening for both mother and baby.1
Common Causes
Placenta previa is not caused by a single factor; rather, several maternal and obstetric conditions increase the risk. The most recognized risk factors include:
- Previous placenta previa – a history of the condition raises recurrence risk to 10‑15 %.
- Prior Cesarean delivery – scarring in the uterus can alter where the placenta implants.
- Multiple pregnancies (twins, triplets, etc.) – the larger uterine cavity provides more surface area for implantation.
- Uterine surgery or fibroids – prior myomectomy, curettage, or congenital uterine anomalies can shift implantation sites.
- Advanced maternal age – women over 35 have a higher incidence.
- High parity – having had several prior births increases risk.
- Smoking – nicotine causes vascular changes that influence placental placement.
- Previous abortions or D&C procedures – similar to other uterine surgeries, they may affect implantation.
- Assisted reproductive technologies (ART) – IVF and related techniques have been linked with higher rates of low‑lying placentas.
- Abnormal uterine blood flow – conditions like pre‑eclampsia can indirectly affect placental positioning.
Associated Symptoms
Many women with placenta previa experience few symptoms until bleeding begins. When symptoms do appear, they commonly include:
- Painless vaginal bleeding – usually bright red and may be intermittent.
- Bleeding after intercourse – sexual activity can disrupt the low‑lying placenta.
- Bleeding after a pelvic exam – digital exams can provoke hemorrhage.
- Uterine contractions – less common, but can accompany bleeding.
- Fetal movement changes – severe bleeding can reduce oxygen delivery, leading to decreased movements.
Note: The amount of bleeding can range from spotting to heavy flow that soaks a pad within minutes. The lack of pain often leads women to underestimate the seriousness of the event.
When to See a Doctor
Any vaginal bleeding after the 20th week of pregnancy should prompt an immediate call to a health‑care provider. Specific situations that demand urgent evaluation include:
- Bleeding that soaks a pad in less than an hour.
- Bleeding accompanied by dizziness, faintness, or a rapid heart rate.
- Persistent bleeding lasting more than 24 hours.
- Reduced fetal movements or a feeling of “quiet” baby.
- Fever, chills, or foul‑smelling vaginal discharge (possible infection).
Prompt medical attention can prevent maternal shock and improve fetal outcomes.
Diagnosis
Diagnosing placenta previa relies on a combination of history, physical examination, and imaging.
1. Medical History & Physical Exam
Doctors will ask about the timing, amount, and triggers of bleeding, as well as prior surgeries, births, and lifestyle factors (e.g., smoking). A pelvic exam is usually avoided in the first trimester because it can provoke bleeding; instead, it may be performed under controlled conditions later if necessary.
2. Transabdominal Ultrasound
The first‑line imaging test is a transabdominal (through the abdomen) ultrasound, which safely visualizes the placenta’s location relative to the cervical os. It can classify previa into four types:
- Complete previa – placenta fully covers the os.
- Partial previa – placenta partially covers the os.
- Marginal previa – edge of placenta within 2 cm of the os.
- Low‑lying placenta – placenta within 2 cm but does not touch the os.
3. Transvaginal Ultrasound
If the transabdominal view is inconclusive, a transvaginal scan (probe placed in the vagina) offers higher resolution and more accurate measurement of the distance between placenta and os.
4. Follow‑up Imaging
In many cases, a low‑lying placenta early in pregnancy migrates upward as the uterus expands. Repeat ultrasounds at 28‑32 weeks are standard to reassess positioning.
5. Laboratory Tests
Blood type and antibody screen, complete blood count (CBC), and coagulation profile are ordered to prepare for possible bleeding and to plan for transfusion if needed.
Treatment Options
Treatment balances maternal safety, fetal maturity, and the severity of bleeding. Management can be divided into expectant (conservative) care and active delivery planning.
Expectant (Conservative) Management
- Pelvic rest: No intercourse, no digital exams, and avoidance of heavy lifting.
- Activity modification: Bed rest or reduced activity during episodes of bleeding; some physicians recommend hospitalization for close monitoring.
- Hospital observation: For moderate bleeding, patients may stay in the hospital for 24–48 hours with intravenous (IV) fluids and fetal monitoring.
- Corticosteroids: If bleeding occurs before 34 weeks, a course of betamethasone is given to accelerate fetal lung maturity (per ACOG guidelines).2
- Blood transfusion preparation: Cross‑matched blood is kept on standby for severe hemorrhage.
Active Delivery Planning
Delivery timing depends on gestational age, bleeding severity, and fetal status.
- Before 36 weeks – Attempt to prolong pregnancy if bleeding is mild and the mother is stable, using corticosteroids and close monitoring.
- At or after 36 weeks – Planned Cesarean delivery is usually recommended to avoid labor‑induced bleeding.
- Severe or uncontrolled bleeding – Immediate delivery (often by emergency C‑section) irrespective of gestational age.
- Tocolytics – Medications such as nifedipine may be used briefly to halt premature contractions, allowing time for steroids to work.
- Uterine artery embolization – In rare, life‑threatening cases, interventional radiology can block bleeding vessels while preserving the pregnancy.
Post‑Delivery Care
After birth, women are monitored for postpartum hemorrhage, infection, and anemia. Most will require iron supplementation and a follow‑up ultrasound to confirm placental expulsion.
Prevention Tips
While placenta previa cannot always be avoided, certain steps may reduce risk:
- Quit smoking before or early in pregnancy.
- Limit unnecessary uterine instrumentation (e.g., avoid elective D&C when possible).
- Plan future pregnancies wisely – discuss the timing of repeat C‑sections and consider a trial of vaginal birth after Cesarean (VBAC) only when medically appropriate.
- Maintain a healthy weight – obesity is linked with several placental disorders.
- Seek early prenatal care – early ultrasounds help identify low‑lying placentas, allowing for monitoring and counseling.
- Use assisted reproduction judiciously – discuss risks with your fertility specialist.
Emergency Warning Signs
- Heavy vaginal bleeding that soaks a pad in less than an hour.
- Sudden weakness, dizziness, fainting, or a rapid, thready pulse.
- Severe abdominal or pelvic pain accompanied by bleeding.
- Signs of shock: clammy skin, pale complexion, shortness of breath.
- Sudden decrease in fetal movement.
References
- Mayo Clinic. Placenta previa. Available at: mayoclinic.org. Accessed June 2026.
- American College of Obstetricians and Gynecologists (ACOG). Practice Bulletin No. 183: Placenta Previa. Obstet Gynecol. 2021.
- Centers for Disease Control and Prevention. Pregnancy complications: Placenta previa. 2022.
- World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience. 2022.
- Cleveland Clinic. Placenta previa: Symptoms, causes, treatment. 2023.