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The placenta is the organ which transfers oxygen and nutrients from the mother’s blood into the baby’s blood. It is connected to the mother’s uterus over a wide surface area. The baby is connected to the placenta via the umbilical cord.
The placenta can be situated anywhere on the surface of the uterus. The front wall is called anterior. The back wall is called posterior. The side walls are called left lateral or right lateral. The top wall is called fundal.
What matters most is how low the lower edge of the placenta extends because if it is too low in the uterus it can prevent the descent of the fetal head during labour. Low-lying placentas can also lead to unusual bleeding during the pregnancy or birth.
Ultrasound is used to determine the location of the placenta and its proximity to the cervix. It can change during the pregnancy for a number of reasons.
- In mid-gestation the placenta occupies 50% of the uterine surface. By 40 weeks' gestation, the placenta only occupies 17 - 25% of the uterine surface. It doesn’t shrink, but the rest of the pregnancy grows more and the uterine surface expands.
- In the third trimester the baby’s head starts to descend into the pelvis in preparation for labour. The pressure of the fetal head on the lower part of the uterus (the lower uterine segment) causes it to stretch and become thinner. The site of the placental attachment then appears to rise.
Because of these reasons many pregnancies have a low-lying placenta at 18-20 weeks gestation, but they do not have a low-lying placenta by the end of the pregnancy.
If the placenta does stay low-lying we then call it placenta praevia.
If the placenta completely covers the cervix then there is no way the baby can deliver vaginally without causing massive haemorrhage from mother and baby.
In this situation Caesarean section is the only safe way to deliver the baby.
Predisposing factors for placenta praevia:
- Advanced maternal age
- Multiparity – having had more than one baby
- Prior cesarean section
- Uterine curettage – after miscarriage or termination
- Maternal cigarette smoking
The risk of placenta praevia increases after previous Caesarean sections:
|Prevalence Placenta Praevia (%)
How low is too low?
There is much discussion about how far away from the cervix the placenta should be to allow a normal vaginal birth without bleeding from mother or baby. It obviously depends on a number of factors, like:
- Whether there are fetal blood vessels below it
- The size of the baby traveling down the birth canal and how much room it needs to get past it
- Whether there has already been bleeding from the placenta
Generally speaking, if the placenta is >2cm from the cervix at the 18-20 week scan, it won’t be low-lying at the time of the birth. It doesn’t need to be rechecked.
But if the placenta is <2cm from the internal os at the 18-20 week scan, it may still be low at the time of the birth. Therefore many obstetricians and mid-wives ask for it to be re-checked in the third trimester.
There is no need to worry unduly about this finding because about 10-15% of placentas are low-lying at the 18-20 week scan. But only 0.5% are still low-lying by full term.
A trans-abdominal image of a placenta with the umbilical cord leading from the placenta to the baby’s belly-button. This placenta is on the front wall of the uterus.
A trans-abdominal image of the cervix and the lower edge of the placenta. The placenta is on the back wall of the uterus and it is 6.2cm from the cervix.
A trans-vaginal view of the placenta and cervix. The placenta is on the posterior (back) uterine wall. It covers the cervix completely.
A trans-vaginal view of the placenta and cervix. The placenta is on the posterior (back) uterine wall. It is 1.5cm from the cervix.