Placenta previa is the leading cause of third-trimester vaginal bleeding. It is defined as the implantation of the placental tissue in the lower uterine segment, adjacent to or overlying the mouth of the womb called the internal cervical os. The incidence of placenta previa is 4 to 5 per 1000 pregnancies.


The exact cause of placenta previa is not known, it is however thought to be multifactorial. One theory explains that placenta previa occur because implantation often take place in fresh site; usually away from scar areas and previous sites of implantation. The following are risk factors for placenta previa;

  • Infertility treatment
  • Increasing maternal age
  • Having two or more deliveries
  • Pregnancy with more than one baby
  • Short inter-pregnancy interval; the duration from delivery to the next conception
  • Previous uterine surgery like myomectomy or D & C procedure
  • Previous caesarean delivery
  • Previous or recurrent abortions
  • Previous placenta previa 
  • Smoking
  • Cocaine use
  • Diabetes
  • Residence at higher elevation


There are four types or degrees of placenta previa based on its relationship with the mouth of the womb also called the cervical os namely;

  • Type I /Low-lying: within 2cm margin from the mouth of the womb.
  • Type II/ Marginal: the placenta reaches the margin of the internal os but does not cover it
  • Type III/Incomplete or partial central: the placenta covers the internal os partially when closed but not when dilated.
  • Type IV/complete: the placenta completely covers the internal os even when dilated.


Placenta previa present as a painless, spontaneous, unprovoked bright red third-trimester vaginal bleeding. This bleeding may trigger uterine activity. There may be dizziness, weakness or fainting if the bleeding is profuse. There is usually a prior history of bleeding early in the index pregnancy. However, with ultrasound scan, most cases of placenta previa are now detected antenatally before the onset of significant bleeding. When this is discovered early in pregnancy, the scan is repeated at 32weeks as there is the possibility that a prior placenta previa will no longer be a previa. The theory of placenta migration explains this; with the formation of the lower uterine segment at 32weeks of pregnancy, the placenta will apparently move from the lower segment.


The complication of placenta previa include;

  • Preterm delivery
  • Abnormal fetal presentation
  • Intrauterine growth restriction
  • Haemorrhage
  • Higher rate of blood transfusion
  • Increased incidence of postpartum endometritis
  • Need for hysterectomy
  • Maternal death.


The management of placenta previa most importantly depends on the clinical state of the patient; whether she is bleeding or not. Other considerations are the gestational age and resolution of bleeding. Asymptomatic patients with placenta previa or patients with a history of a small bleed that has resolved and remote from term are potential candidates for conservative management.

Conservative management consist of a hospital admission for bed rest usually in a ward with close proximity to the theatre. This is so because, there is a danger of a second bleeding which is usually heavier. When it happens, intervention will be prompt. The first bleed is consider the warning bleeding. At all-time while on admission, 4pints of blood should be readily available in the blood bank in case of emergency.

The conservative management will be discontinued when; the pregnancy gets to 37weeks, bleeding becomes torrential, when the baby’s water breaks, when labour starts, when the baby is in distress or it growth in the womb is restricted.

For asymptomatic patients with placenta previa, delivery is typically recommended by 37weeks gestation with documented fetal lung maturity or after administration of antenatal corticosteroids. The timing of delivery depends on clinical circumstances. Any patient with persistent haemorrhage mandates delivery regardless of gestational age. Caesarean delivery is indicated for all patients with sonographic evidence of placenta previa. In women with a low-lying placenta vaginal delivery may be considered.