Ultrasound to check the position of the placenta

Is a low-lying placenta a problem?

It depends on the stage of your pregnancy.

A placenta is low lying if it's near, touching, or covering your cervix inside your uterus (womb).

If your placenta is found to be low lying in a scan during the first half of your pregnancy, it's usually nothing to worry about. In most cases, the placenta moves up as the pregnancy progresses (RCOG 2018).

However, later in your pregnancy, a low-lying placenta can be a problem because:

• It may block the opening of the cervix inside your uterus (cervical os) and prevent your baby from being born vaginally. This is called placenta previa (NHS 2018, RCOG 2018).
• As the lower part of the uterus stretches in the second half of pregnancy, the placenta may become detached, causing severe bleeding (Rowe 2014). This is called placental abruption. It's a serious complication but, thankfully, it's also rare (Ananth et al 2015).

It's important to distinguish between placenta previa and placental abruption. Although both can cause bleeding in pregnancy, they are very different conditions.

Placenta previa is more likely if you:

• have weakness in the wall of your uterus, due to surgeries such as one or more cesareans
• have had fertility treatment such as IVF to help you conceive
• have uterine fibroids
• smoke
• are over 35
• are carrying twins or more (Bakker 2016, Oppenheimer 2007, Rowe 2014, RCOG 2018)

Even given these quite common reasons, in North America, placenta previa still affects only about one pregnancy in 350 at full term (Oppenheimer 2007, Rowe 2014).

We don’t know what causes placental abruption, but it is more likely if you:

• had one in a previous pregnancy
• are a teenager, or 35 or older when you give birth
• are carrying twins or more
• have high blood pressure
• are a smoker
• use cocaine
• have had a trauma such as a road traffic accident (Ananth et al 2015, RCOG 2011, NHS 2018)

In some cases, placental abruptions can be "silent", giving you a dull ache and very light bleeding. If you have these symptoms you should contact your doctor or midwife immediately (NHS 2018).

How will I know if I have a low-lying placenta?

Your healthcare provider will tell you about the position of your placenta when she talks you through the results of your anatomy scan (Cargill and Morin 2017). If your placenta is too low, you may be offered another ultrasound in the third trimester of pregnancy, usually when you’re around 32 weeks pregnant. The scan may be performed via the vagina, as it’s more accurate in this situation (Oppenheimer 2007, RCOG 2018). This will only take a few minutes and will clearly show how close the placenta is to your cervix.

By this time, for nine out of 10 women, the placenta will have moved up and out of the way of the cervix (NHS 2018, RCOG 2018, Rowe 2014).

If your placenta's still low, you may need another ultrasound a few weeks later (Oppenheimer 2007, RCOG 2018).

What happens if my placenta is too close to my cervix?

More than a third of placentas are low in the first trimester, but very few are still low at the end (Oppenheimer 2007). So it's unlikely that you'll end up with placenta previa.

If your anatomy ultrasound shows that you have a low-lying placenta, your healthcare provider may advise you to avoid sex (RCOG 2011). You should also alert any other healthcare providers you see, in case you need a vaginal examination (RCOG 2011).

Sometimes, placenta previa causes painless, bright red bleeding from the vagina during the last three months of pregnancy. If you have these symptoms you should contact your healthcare provider right away (NHS 2018, Oppenheimer 2007, RCOG 2018).

If the placenta's still low when your baby's ready to be born and is overlapping the entrance to the cervix, your obstetrician will advise you to give birth by cesarean section (Oppenheimer 2007).

Do I have to have these ultrasounds?

No ultrasound scan is compulsory, so it's up to you whether you have this one. But it might reassure you to know that ultrasounds are considered to be safe (van den Hof 2018), and are the best way to check that the placenta is working properly to support your baby (Oppenheimer 2007, RCOG 2018). It’s also an opportunity for your healthcare practitioner to check on the development of your baby.

If you do have a low-lying placenta, find out more about placenta previa.