Placenta praevia (aka.
placenta previa) is a condition in pregnancy where the placenta is low-lying.
The placenta is your baby’s
‘life support’ system during pregnancy. It is shaped a bit like a pancake
attached to the inside of your womb (uterus), and your baby’s umbilical cord
emerges from the centre. The cord is like a lifeline with blood going in two directions.
A placenta is considered to
be low-lying when it is lying in the lower part of the uterus near, or over,
the opening of the womb (cervix). Most low-lying placentas are near but not
covering the cervix. If your placenta is partially or completely blocking the
womb opening at the end of pregnancy, this is known as known as ‘placenta
praevia’ (pronounced ‘preev-ee-ah’). In these cases, your baby will need to be
born by caesarean section.
However, a low-lying
placenta in early or mid-pregnancy will most often have moved out of the way by
late pregnancy.
During labour the
cervix opens to let the baby move down into the vagina and be born. If the
placenta overlaps the cervix, the placenta will begin to separate as the cervix
opens, causing bleeding, usually from the mother. Bleeding can be extensive and
can occasionally lead to shock if it is not treated promptly.
At your 18-21-week ultrasound scan your placenta’s position will be
recorded. If your placenta is significantly low, you’ll be offered an extra
scan to check again, usually at about 32 weeks.
The vast majority of women
who have a low-lying placenta in the middle of pregnancy will not be affected
in the later weeks. At the end of pregnancy, only a tiny percentage of women
have the condition. One study of 6,428 women found that only around 1 in 600
had placenta praevia at the time of birth. In most cases the placenta, even if
relatively low in the uterus, will not be affected by the cervix opening during
labour.
As your pregnancy
progresses, the uterus grows, particularly the lower part of the uterus, so the
placenta moves with the growing uterus away from the cervix.
It does seem that screening
by ultrasound is often not very accurate and low-lying placenta may be
over-diagnosed.
First, if the placenta is
not actually covering the cervix at 20 weeks of pregnancy, it is unlikely that
there will be a problem at a later stage. Second, the report of evidence used
to inform the Antenatal Care guideline for the NHS says, ‘Many placentas that
appear to cover the cervical os (the opening in the cervix inside the womb) in
the second trimester will not cover the os at term’.
This is partly because as
the weeks go by and the lower part of the uterus grows and stretches, the
placenta appears to move away from the cervix.
In England and Wales, the
Antenatal Care guideline recommends that as ‘most low-lying placentas detected
at the anomaly scan will have resolved by the time the baby
is born, only a woman whose placenta extends over the internal cervical os
should be offered another transabdominal [from the abdomen] scan at 32 weeks.
If the transabdominal scan
is unclear (at 32 weeks), you should be offered a scan via the vagina,
according to these guidelines. An ultrasound scan through the vagina is more
accurate in diagnosing the position than a scan taken from the abdomen.
More information at www.nct.org.uk/pregnancy/low-lying-placenta