In most normal pregnancies, the placenta implants high, in the top of the uterus (called the 'upper segment' or fundus). It is normal for the placenta to implant towards the back of the upper segment (posterior), towards the front of the upper segment (anterior) or on the right or left sides of the upper segment (lateral).
Location of the placenta
If you have an ultrasound after the 16th week of pregnancy, the report usually comments on where the placenta is situated. For example, "posterior placenta, not low-lying".
In about 5 per cent of routine 18 - 20 week pregnancy ultrasounds, the placenta is seen to be lying low. Meaning the placenta is situated lower in the uterus, closer to the woman's cervix. A low-lying placenta at this stage of pregnancy is regarded as normal because the lower segment of the uterus has not fully formed yet.
The lower segment of the uterus starts forming at around the 24th week of pregnancy, and is fully present by 36 weeks. During this time, the uterus slowly stretches upwards, forming the thicker upper segment and leaving a thinner layer of muscle below it, known as the lower segment. The lower segment separates the cervix from the upper segment and is the part of the uterus that absorbs the cervix as it dilates during labour.
In about 90 per cent of cases, the previously low-lying placenta moves up into the upper segment as the uterus grows, meaning that by about the 34th week it is no longer low-lying.
Placenta previa explained
On rare occasions (0.5 per cent of pregnancies), the placenta remains in the lower segment of the uterus, sometimes growing to the extent that it covers the woman's cervix. This is known as Placenta Previa, meaning 'placenta first'. Some factors commonly associated with placenta previa include:
Twins or triplets (or more).
Large fibroids in the uterus.
A previous Caesarean operation or other major uterine surgery.
An unusually formed uterus, say with a septum (called a bicornuate uterus).
Assisted conceptions, such as IVF.
If an ultrasound at 18 to 20 weeks shows the placenta is low, the caregiver often recommends having a repeat ultrasound at 34 to 36 weeks of pregnancy, to check if the placenta has moved up, away from the cervix.
NOTE: Ultrasounds are not 100 per cent accurate and it can sometimes be difficult to tell if a placenta is actually low-lying or not. That is, an ultrasound may question the presence of a low-lying placenta, even though it is situated normally, or the ultrasound cannot tell that the placenta is low lying. Ultrasounds done through your belly are generally regarded as less accurate at diagnosing placenta previa, because the cervix is situated very low down in the woman's pelvis. Ultrasounds done with a probe into the woman's vagina are suggested as being more accurate, but are not generally recommended if placenta previa is suspected, as any vaginal examination could stimulate bleeding.
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