Signs of placenta previa
Types of placenta previa
Health concerns and placenta previa
Care and treatment for placenta previa
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). 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% 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% 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.
Image 1-05 shows the placenta lying in the fundus of the uterus at 16 to 18 weeks of pregnancy.
Image 4-40 shows the placenta lying low in the uterus at 16 to 18 weeks.