Differential Diagnosis
- Single most likely diagnosis
- Placenta previa: The ultrasound finding of the lower edge of the placenta being 3 cm from the internal cervical os is consistent with a low-lying placenta or placenta previa. Although the patient is asymptomatic, this condition can cause painless vaginal bleeding in the second or third trimester.
- Other Likely diagnoses
- Cervical insufficiency: The cervical length of 1.8 cm is shorter than the normal threshold of 2.5 cm, which increases the risk of preterm labor and cervical insufficiency.
- Breech presentation: The fetus is in a breech presentation, which may be a normal variant at 20 weeks gestation but could also be related to other factors such as placenta previa or uterine anomalies.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Placental abruption: Although the patient has no symptoms of vaginal bleeding or abdominal pain, placental abruption can be a life-threatening condition for both the mother and the fetus if not promptly diagnosed and treated.
- Uterine rupture: Although rare, uterine rupture can occur in women with a history of prior uterine surgery or other uterine anomalies, and can be catastrophic if not promptly recognized and treated.
- Rare diagnoses
- Uterine anomaly (e.g., unicornuate uterus): A rare congenital anomaly of the uterus that can increase the risk of obstetric complications, including preterm labor and cervical insufficiency.
- Vasa previa: A rare condition where fetal blood vessels are located in the membranes covering the cervix, which can cause severe fetal bleeding if ruptured.