Low Lying Placenta at the 18-20 week Ultrasound Examination
Normally the placenta is situated in the upper part of
the uterus (womb) or along the front or back wall, well clear
of the birth canal (cervix) see figure 1.
In 5% of pregnancies at 18-20 weeks, the placenta is situated
low in the uterus either close to, or covering the cervix
(see figure 2). As pregnancy progresses however, the lower
part of the uterus grows such that the placenta eventually
becomes clear of the cervix. In fact, in 95% of cases the
placenta will be clear of the birth canal by 37 weeks of pregnancy.
Therefore if you are informed at your 18-20 week scan that
the placenta is low, do not be alarmed, you can continue normal
activities as usual. A repeat ultrasound should however be
arranged at around 26-34 weeks of pregnancy to determine if
the placenta has moved away from the cervix.
In a small number (5%) of pregnancies, the placenta remains
low after 28 weeks. This is called placenta praevia (refer
figure 3). A placenta which is praevia may cause vaginal bleeding
in later pregnancy. Also in some patients with Placenta Praevia
the baby may need to be delivered by Caesarean section.
So in this situation extra care and monitoring of your baby
and the placenta may be required. Your specialist will discuss
with you the best way to manage this situation and advise
you of any precautions.
A normally situated placenta
Figure1. The placenta is situated at the top of the uterus
well clear of the neck of the womb (cervix).
A low placenta at 18 weeks
Figure 2. The placenta is situated low in the uterus. In
95% of patients however the placenta "moves up" in later pregnancy.
Placenta Praevia
Figure 3. The placenta covers the birth canal (cervix).
of the womb (cervix)
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