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Five main clinical situations can cause birth to occur before 33 weeks:

  • High blood pressure and complications
  • Pre-eclampsia
  • Bleeding (hemorrhaging)
  • Early rupture of membranes
  • Spontaneous early labour
  • Intrauterine growth restriction  

Maternal hypertension (high blood pressure)

Hypertension occurs when systolic blood pressure is equal to or higher than 140 mmHg and/or diastolic blood pressure is equal to or higher than 90 mmHg. Usually, we distinguish women who have known hypertension (that existed before pregnancy) from women who develop hypertension for the first time during pregnancy, which is more often the case. 

Frequency: High blood pressure is present in about 20% of deliveries prior to 33 weeks.

Complications: High blood pressure can be complicated by other health issues, the most common being:

  • Pre-eclampsia: Hypertension is associated with the presence of protein in the urine, a sign of kidney problems.
  • Eclampsia: This rare but serious complication of hypertension causes convulsions that signal brain damage.
  • Hellp syndrome: A serious complication of hypertension during pregnancy, Hellp syndrome causes liver disorders and destroys red blood cells and blood platelets. 
  • Retroplacental hematoma: This condition causes the placenta to detach prematurely from the uterine wall and a hematoma to form.


Defined as abundant bleeding that endangers the mother and child, hemorrhaging includes:

  • Placenta previa: A poorly inserted placenta (near or blocking the cervix) can be complicated by hemorrhaging.
  • Retroplacental hematoma: Can occur with hypertension (see above), but also in women with no known history of hypertension.
  • Other hemorrhaging occurring in the third trimester of pregnancy (bleeding whose cause is not always known).

Frequency: Hemorrhaging occurs in about 20% of deliveries before 33 weeks.

Premature rupture of membranes (PROM)

Premature rupture of membranes occurs when the amniotic sac holding the water breaks before full term and labour. One theory to explain PROM blames infection. Currently, it is estimated that one third of PROM cases are linked to an intrauterine infection (chorioamnionitis).
Other factors such as unfavourable socioeconomic conditions and smoking also play a role. PROM is involved in about 25 to 35% of births before 33 weeks.

Spontaneous premature labour

This term describes labour that begins before the normal pregnancy term (37 weeks), although the membranes are still intact (mother’s water has not yet broken when labour begins). Once again, infections are suspected of playing a role. At least 15% of women giving birth after spontaneous premature labour have been shown to have a uterine infection (chorioamnionitis). Other events, especially difficult living conditions and stress, are also involved. Study results show that spontaneous premature labour accounts for about 25 to 30% of births before 33 weeks.

Intrauterine growth restriction

This term is used when the birth weight is too low for the baby’s gestational age. Growth curves (such as those found in the health booklet) for gestational age are used to identify the condition. Healthcare professionals can thus spot children who are not growing sufficiently in the womb. Growth delay is generally diagnosed or detected thanks to ultrasound during pregnancy (based on the growth curves). Growth delay is partly linked to abnormal blood flow between the uterus and the placenta. Since the maternal-fetal link has been weakened, the supply of nutrients and oxygen is insufficient. If the placenta is too unhealthy and the supply very inadequate, the fetus will be at serious risk. In this situation, medical teams will induce premature birth, most often by caesarean section. The link between maternal blood pressure and a growth delay in the child is therefore very common. Other causes, such as some fetal conditions (malformations or genetic disease) can worsen intrauterine growth restriction.