The Scary Box - Placenta Previa - [TW] Pregnancy Complication

By: Aunt Doula (01/17/2023)

Placenta Previa is a pregnancy condition diagnosed at differing stages of pregnancy at regular prenatal ultrasound scans. If you imagine your cervix which is the bottom part of your uterus as a exit hatch that must be kept clear at all times, and the placenta has taken up a parking space near, partially or directly on the hatch, this is placenta previa.

Placenta previa is typically seen early in the second trimester at the 20 week anatomy scan. It is important to note first and foremost, the placenta can "move" itself up the uterine wall, and in most cases it does. There are three primary situations of placenta previa:

1) Low-Laying placenta that is near the edge of the cervix but not overlapping the cervix

2) Low-Laying placenta covering a portion of the cervix usually denoted by a percentage or mm measurement.

3) A 'complete placenta previa' where the embryo implanted directly on or near the surface of the cervix and the placenta has grown to entirely cover the cervix and is blocking the cervical 'exit hatch.' In this case, the exit is blocked and vaginal delivery is impossible.

When there is a placenta previa detected, it is typically found as scenario 1 or 2 and usually by week 32 it has moved to a safe distance away from the cervix on its own and there are no concerns for delivery. This is because in the lower parts of the uterine wall, there are less abundant blood vessels to feed the placenta and so it "seeks" better blood supply and moves upward toward the top or one side of the uterus. The placental edge ideally needs to be over 20mm away from the internal cervical os (opening of the cervix) in order to proceed with vaginal delivery, and most pregnant people experience this outcome.

In situation #3 the placenta may not move significantly because it taps into the blood supply of the cervix which is very ample and is capable of sustaining a pregnancy decently well. When the placenta is just low-lying or only partially covering the os, it usually starts to inch its way upward to the higher areas of the uterus where there are larger blood vessels for it to connect with; without this drive to seek a stronger blood supply the complete placenta previa is the least likely to resolve itself because it generally has what it needs and no reason to seek out a better source.

There is no treatment for placenta previa - it will either move or it won't. It may move some, but not enough. It is difficult to predict which placentas will move more than another, so it is a watch & wait scenario. If by 32 weeks your placenta is still low laying and has not moved the requisit 20mm from the cervical os, you will very likely be counselled on having a C-section. There are edge cases (what if it's 19mm?) and it is strongly urged that you to discuss the benefits and risks if you are close but not quite in the zone. Some providers have lower thresholds, but the given recommendations at the time of this post are that at 0-10mm distance a C-section is the standard of care, from 10mm-20mm it is not advised to go through with a vaginal delivery however you may be able to discuss with your provider about your wants, needs, and feelings on the matter.

The usage of a C-section is to prevent sudden catastrophic fetal & maternal hemorrhage (blood loss). The placenta is the organ that is providing the baby with many things, but the most important thing during labor until they take their first breath, is oxygen rich blood. If the placenta is too close to the cervix during labor, it can be torn or damaged as baby moves down into the pelvis and it can compromise their blood supply (as such, their oxygen supply) and they would begin bleeding as would the birthing person and this puts both in grave danger. In cases of complete previa, a pregnant person is often scheduled for a C-section closer to 37 weeks to avoid any changes to the cervix and all labor processes which could disturb and tear the placenta due to dilation which could trigger the catastrophic bleeding. It is exceptionally important to follow all the guidelines your provider gives you in regards to activity and pelvic rest if it given. Low laying placentas are less likely to need activity restrictions, however a complete previa may require special instructions to keep the placenta protected and undisturbed. If a previa is disturbed, its most likely manifestation is a placental abruption which is its own emergency and is covered in its own post that you can click the blue text to be taken to.

Causes of placenta previa are not well understood. It can happen randomly when the embryo is implanting it goes farther down toward the lower uterus before it fully seats into the lining, be disturbed by fibroids or scarring of the uterus but ultimately once the embryo has implanted, that is where it will grow. Biologically because the placenta can move, it is not considered a "problem" until 32 weeks without sufficient movement to the 20mm distance. 32 weeks is chosen because the "fastest" documentation of placenta movement was just above 4mm per week and with 5 weeks to the scheduled C-section week for previas under 20mm, 32 weeks is when the planning begins with the theory that it's not very likely your placenta will suddenly get a move on and hit the 4mm per week for 5 weeks and reach 20mm distance for a safe vaginal delivery.

If a vaginal delivery is extremely important to you, start talking with your provider early about monitoring until week 32, following protocols for scheduling the 37 week C-section but getting an ultrasound 1 or 2 days before the C-section to see if your placenta did move enough. This way you can feel most confident in your knowledge that you tried to wait as long as possible and do as much as you could for your desired birth. At the same time during those 5 weeks, be mentally preparing yourself as if you will be getting a c-section - be prepared for both outcomes in their entirety to the best of your ability.

There may be some extra precautions put in place when you arrive at the hospital such as type & cross testing your blood in case you need a rapid blood transfusion, and a neonatal team may be present for the birth instead of only L&D nurses and doctors. It is also possible that they will have you give birth in a fully prepared operating room to be able to convert to surgery immediately if there were any bleeding problems. It is strongly recommended that you ask these questions and know what happens if things go that way so you are not left unknowing and out of the decision loop. The more you know about what may happen means that if it does happen, you have some semblance of what to expect. That is a cornerstone of empowerment in your pregnancy, labor, and delivery.

The long and short of Placenta Previa is, until 32 weeks, chances are that your placenta will move if they are presenting as scenario 1 or 2. Scenario 3 may resolve but it is less likely and you may have more restrictions placed on your activities throughout pregnancy and a C-section at 37 weeks. Ask lots of questions at your appointments about where your placenta is in relation to the cervical os, and what the distance must be in order to qualify for the vaginal birth. Ask how they will ensure you and your baby's safety in any situation with a placenta previa in the mix. You've got this.