Low Lying Placenta and Placenta Previa

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How does the location of my placenta impact my pregnancy and birth?

Early identification of low-lying placenta and placenta previa has recently become possible due to the increased detail of first and second-trimester ultrasounds. The location of the placenta during pregnancy is important to know in its relationship to the internal os of the cervix (the internal opening of the cervix).

  • Normal placenta: is attached to the uterine wall greater than 2 cm away from the internal cervical os.
  • Low lying placenta: is attached to the uterine wall less than 2 cm away from the internal cervical os.
  • Placenta previa: is the partial or complete covering of the internal os by the placenta.

Approximately 90% of low-lying placentas identified in the first and second trimesters will resolve secondary to placenta migration (movement of the placenta up and away from the cervix as the uterus grows with the pregnancy) (Anderson & Sze, 2022). Only 0.3% to 2% of pregnancies have persistent placenta previa in the third trimester (Anderson & Sze, 2022).

placenta previa diagram

Risk Factors for Developing Placenta Previa

  • Client age greater than 35 years old
  • Multiparity (already given birth to at least 1 baby)
  • History of cesarean section
  • Smoking
  • History of curettage
  • Use of cocaine

Identifying and Monitoring of Placenta Previa

The majority of placenta previas and low lying placentas will be identified at the 20 week anatomy ultrasound. However, another symptom may be painless vaginal bleeding during the second or third trimester. An official diagnosis of placenta previa or low lying placenta can not be made until the third trimester. This is because as the uterus grows the placenta generally moves up and away from the cervix. If there are concerns about your placenta’s location, your midwife will order an additional ultrasound around 32 weeks in the third trimester (SOGC, 2020). If placenta previa or low lying placenta is seen on this third trimester ultrasound, then a diagnosis will be made and you will continue to be monitored for the duration of your pregnancy. If your placenta has moved into a normal position by the third trimester, no further monitoring is required.

If you have been diagnosed with placenta previa or low lying placenta in the third trimester, your midwife will consult with an obstetrician (OB). If indicated, your baby may require birth by cesarean section. This will depend on the specific measurements and location of your placenta.

What Can I do?

The location of the placenta only changes with the growth of the uterus. This means there is nothing clients need to or can do to impact placental location. Just be patient and wait for the third-trimester ultrasound. Remember that 90% of low-lying placentas will move up into a normal safe position by the third trimester.

As there is an increased chance of bleeding with placenta previa and low-lying placenta, your midwife will provide you with a list of precautions to follow while you are waiting for your third-trimester ultrasound.