If a placenta is ‘low-lying’ early in pregnancy, it simply means that part or all of the placenta is developing in the lower part of the uterus. Now that ultrasound scans seem to be routine, as many as one in four women are told that their placenta is ‘low lying’ – yet having a ‘low-lying’ placenta at 20 weeks does not necessarily mean that your placenta will still be low later in pregnancy. This is because the lower segment of the uterus grows by several centimetres in the later part of pregnancy, ‘carrying’ the placenta upwards with it. Fewer than 1 per cent of pregnancies are complicated by a major placenta previa (low-lying placenta) at 40 weeks.
The problem with the term ‘low-lying placenta’ at 20 weeks of pregnancy is that it is (unavoidably) very vague. If the placenta is low in the uterus, but not actually touching the cervix (neck of the womb), then sexual intercourse is unlikely to cause problems.
On the other hand, a placenta that appears to touch or cover the internal os (top part of the cervix) may be disturbed by sex, and this may cause serious bleeding.