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Placenta Previa and the Magic of Placental Migration.

It’s never fun to hear that your placenta is in the wrong spot. Honestly, I didn’t even know that was a possibility until my 20 week ultrasound when the sonographer told me I had a low lying placenta. In her words: “Not technically placenta previa but your placenta is very close to your cervical opening and that’s not good.”

Without much more explanation (even though I put on my best “I have no idea what you’re saying right now” face) she continued:

“Oh, don’t worry about it, the placenta usually moves away from the cervix as the uterus grows.  And if it doesn’t move, you’ll just have a C-section”.

Ok, personally, I don’t consider major surgery an “Oh, don’t worry” type of situation. And, honestly, I was not very happy with the limited information she could offer about what the heck she meant by low-lying placenta, placenta previa, or even “it usually moves away”. As I’m wont to do, I started digging…



Low lying placenta:

Defined by the location of its edge being < 2 cm from the opening in the cervix (baby’s escape hatch) but not covering any part of the cervical opening. Further defined by location within the uterus, either attached to the front (anterior low-lying placenta) or the back (posterior low-lying placenta).

Placenta Previa:

The more serious condition. The placenta is covering the cervical opening. Different types of placenta previa are defined by the degree of overlap of the cervix: either complete placenta previa and incomplete placenta previa.  Complete placenta previa is the worst case scenario.  A placenta blocking the birth canal can result in hemorrhage and other scary outcomes, which can be avoided by C-section.

Here is one mom’s story about how she was able to have a family-friendly, gentle c-section with placenta previa.


A bit side-tracked in my research, I found that I could decrease my risk of having a low-lying placenta or placenta previa by:  1) not smoking (check);  2) being younger (hmm… can’t do much about this one);  3) not having a previous c-section (check); and 4) avoiding crack (big time check).

Basically, in my case, there wasn’t anything that I could have done to avoid this moment.  Now that I answered the basic questions, the last point got stuck in my head: “it usually moves away”.

What does that mean?!

The magical migrating placenta

Reflecting back, I’m guessing my sonographer struck a pretty relaxed tone because low-lying placenta as a diagnosis really isn’t as scary as it sounds. A recent study looked at nearly 1500 cases of low-lying placenta diagnosed in the second trimester and found that over 98% had resolved before delivery. Most (84%) had cleared before 32 weeks. The rare cases that didn’t resolve (the extra 1.6%) delivered their babies via c-section with no further complication[1].

When I asked my nurse how this happens she explained it this way: as the uterus grows, the placenta will go with it, moving up and away from the cervix.

In my mind I pictured my badass placenta, catching a ride on the expanding uterus – “So long, cervix!” But after doing a bit more digging another word popped out: migrate [2].

Ok, so good chance my placenta would move and I’d be fine but now I’m curious… the placenta migrates?!


Over 98% of low-lying placenta cases resolved before delivery.


I now had two questions to answer:

1) What does placental migration even mean?  

2) What is the likelihood that my anterior low-lying placenta will migrate all on her own?

Placental migration

This is where things get nerdy. But cool. So cool.

Placentas can migrate “passively” or “actively”. Passive migration occurs when the placenta simply follow the growth of the uterus (catching a ride). Active migration occurs when the placenta actively responds to changes in the uterine environment[3]. How this works:

Dynamic Placentation:

Placental attachment points are constantly forming and re-forming [4]. Dynamic placentation describes how this rearrangement responds to both uterine growth and placental growth. When the lower part of the uterus stretches and the wall thins, the attachments in this area to degrade.  As those attachments degrade, new attachment points are formed higher in the uterus in areas that are not subjected to this same kind of growth stress. The placenta creeps along through growth, degradation and re-formation.


Trophotropism is often lumped in with active dynamic placentation when it comes to placental migration with a few, fascinating specifics (I warned you that things would get nerdy).

Picture the placenta as a sunlight-seeking plant – just as plants need sun, placentas need a maternal blood supply.  Phototropism is when plants bend to bask in the best sun beam. Trophotropism is when the placenta moves to find the best blood supply[5].  

As a uterus grows the bottom portion of the uterine wall stretches and, as a result, the blood supply at the bottom of the uterus thins.  The placenta starts to seek greener (bloodier) pastures and moves away from this thin blood supply towards the thicker, upper uterine wall.  End result: the placenta moves up and away from the cervix.

“The concept of ‘trophotropism‘, which, in an oversimplification, can be phrased as ‘the placenta grows where it can, and does not grow where it cannot’.” – Salafia et al. 2012

Placental migration via trophotropism also helps explain why centrally located placenta previa often will not end up migrating during the pregnancy[6].  The cervix itself has a healthy blood supply – the placenta has no reason to move.

Which placentas will migrate?

A 2008 analyzed placental migration for every flavor of placenta: anterior low-lying placenta, posterior low-lying placenta, incomplete placenta previa, and complete placenta previa[6].

Anterior low-lying placenta: In the 28 out of 29 studied cases the placenta migrated away from the cervix. The one anterior low-lying placenta that didn’t budge also did not require a C-section at term.

Posterior low-lying placenta: 36 out of 40 posterior low-lying placentas migrated out of the away.

Incomplete placenta previa: 15 out of 22 placentas partially covering the cervix migrated by term.

Complete placenta previa: only 1 out of 7 posterior complete placenta previa cases migrated.  

More importantly, who would win in a placenta race?

The winner is: anterior incomplete placenta previa! Coming in with the swift speed of 4.1mm/week.

For comparison, the slow pokes include anterior low-lying placenta at 2.2 mm/week and posterior low-lying placenta at 1.4 mm/week.


The winner of the placenta race:

Anterior incomplete placenta previa!

Coming in with the swift speed of 4.1mm/week.


In my case, my anterior low-lying placenta did move away from my cervix. I got an all clear at my 28 week ultrasound and I delivered vaginally without any concerns.

(Side note: a follow-up study showed that C-section scars do not affect the rate of placental migration[7].)

One last fun fact:

How does the little ball of cells know where to implant itself?

While there are quite a few studies trying to get to the bottom of this, one possible cause: good ‘ol gravity [8].  In one study, the researchers found that women who preferred to sleep on their right side, were more likely to show right-sided placentas.  The opposite held true for the left-sided sleepers.  The findings were published in Military Medicine and the goal of the study was to understand the effects of zero gravity insemination.

I guess we can add this one to the list of things to decrease the risk of a misplaced placenta:  

6) don’t get pregnant in space.


Next on your reading list: 


[1] Heller HT, Mullen KM, Gordon RW, Reiss RE, Benson CB. Outcomes of pregnancies with a low-lying placenta diagnosed on second-trimester sonography. J Ultrasound Med. 2014;33: 691–696.

[2] Rizos N, Doran TA, Miskin M, Benzie RJ, Ford JA. Natural history of placenta previa ascertained by diagnostic ultrasound. Am J Obstet Gynecol. 1979;133: 287–291.

[3] Salafia CM, Yampolsky M, Shlakhter A, Mandel DH, Schwartz N. Variety in placental shape: when does it originate? Placenta. 2012;33: 164–170.

[4] King DL. Placental migration demonstrated by ultrasonography. A hypothesis of dynamic placentation. Radiology. 1973;109: 167–170.

[5] Benirschke K, Kaufmann P. Pathology of the Human Placenta. Springer Science & Business Media; 2013.

[6] Cho JY, Lee Y-H, Moon MH, Lee JH. Difference in migration of placenta according to the location and type of placenta previa. J Clin Ultrasound. 2008;36: 79–84.

[7] Naji O, Daemen A, Smith A, Abdallah Y, Bradburn E, Giggens R, et al. Does the presence of a cesarean section scar influence the site of placental implantation and subsequent migration in future pregnancies: a prospective case-control study. Ultrasound Obstet Gynecol. 2012;40: 557–561.

[8] Magann EF, Roberts WE, McCurley S, Washington W, Chauhan SP, Klausen JH. Dominant maternal sleep position influences site of placental implantation. Mil Med. 2002;167: 67–69.

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Molly Dickens, PhD

About Molly

Molly has her PhD in Physiology and spent over a decade as an academic research scientist slightly obsessed with the colliding worlds of brain science, hormones, stress and the reproductive system. Nowadays she heads up Content and Community at Bloomlife and edits Preg U. Science is still her jam and she can't help but continue to dive into the research world to find interesting bits about pregnancy and parenting.


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