Category: fun with placentas

About that placenta…

De formato foetu liber singularis (1626) [1]     

I spent last Monday morning on a doctor’s table with goo on my belly, spying on the womb baby.
We found out the sex and about this baby’s love for doing flips and refusing to sit long enough in profile to get a money shot for the grandmas.

We also found out that I have a low-lying placenta.

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

What?  It’s not a big deal, we’ll just cut the baby out?

Sorry, I don’t take that as reassurance.  So, of course, I did a little bit of reading.

First, definitions – A low-lying placenta is defined by the location of its edge being < 2 cm from the opening in the cervix (baby’s escape hatch).  A more serious condition is when the placenta is actually covering this opening; this is called placenta previa.  It comes in two flavors – complete and incomplete.  The terminology specifies the degree of overlap of the cervix with complete placenta previa being 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.

So, back to my selfish scenario – what about me?  What are my risks?

Hitting up the always helpful internet for advice, I found that I could decrease my risk of having a low-lying placenta and 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) not hitting the crack pipe (big time check).

Not helpful. Plus, it is too late anyway.  The little bugger has already set up shop in this uterus of mine, now it just has to move its blood supply bag out of the way.

The sonographer and my nurse told me that as the uterus grows, the placenta will go with it, moving up and away from the cervix.  Reassuring, yes, but catching a ride on the expanding uterus might not be the true travel mode for the placenta.  Rizos et al. in 1979 note that 90% of placenta previa diagnosed during the 2nd trimester actually migrate to a normal location by term [2].

Wait, migrate?  The placenta can move on its own!?

My research mission:  1) what the hell does placental migration mean?  and 2) What is the likelihood that my anterior low-lying placenta will jump on this train and get out of the way by the time this baby wants out?

Placental migration –  So, we don’t really know how the placenta is doing this magic move technique, but there are are few hypotheses.  The two that rise to the top are the Dynamic Placentation hypothesis and the Trophotropism hypothesis.

Dynamic Placentation is based on the idea that placental attachment points are constantly forming and re-forming [3]. The rearrangement is in response to uterine growth as well as placental growth such that as the lower uterine wall forms and the muscles stretch, the stress causes 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 – ok, brace yourself, this one is pretty cool (if you are a huge nerd like me).  Picture the placenta as a plant seeking sunlight – 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 migrates to find the best blood supply [4].  A growing uterus means that the bottom portion of the uterine wall gets stretched and, as a result, the blood supply thins.  The placenta then seeks greener pastures, moving away from this thin blood supply and towards the thicker upper uterine wall.  And here we have the same effect – over time, the placenta moves up and away from the cervix.

A different research group suggested that the trophotropism hypothesis would explain why centrally located placenta previa (sitting smack on the cervix) often do not end up migrating during the pregnancy [5].  The cervix has a healthy blood supply – why move?

So what is the prognosis for my anterior (in the front) low-lying placenta?
A study by Cho and colleagues found that, for the studied cases of anterior low-lying placenta and incomplete placenta previa, nearly all cases showed placental migration away from the cervix (28 out of 29) [5].  And the one anterior low-lying placenta that didn’t budge, did not require a c-section at term. The migration, however, was not as strong in those placentas located posterior (in the back).  Migration was noted for 1 out of 7 posterior complete placenta previa cases, 15 out of 22 for incomplete placenta previa, and 36 out of 40 for posterior low-lying placentas.  These data suggest that there is a pretty good chance that my front-sitting placenta is already on the move.


But more importantly, who would win in a placenta race?  Anterior incomplete placenta previa takes that medal 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.

While I am planning to save the deeper wonders of the placenta for a later post, I wanted to throw in one more tidbit – how the hell does the little ball of cells know where to implant itself?

There are many studies addressing this (and I hope to include them in that later post) but one simple study suggests that we might have good ‘ol gravity to blame [6].  The authors 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 authors, publishing their findings in Military Medicine, were very concerned about zero gravity insemination.

Add this one to the list of things to decrease the risk of a misplaced placenta – 5) don’t get pregnant in space.



1De formato foetu liber singularis (1626)

2Rizos N, Doran TA, Miskin M, et al. (1979) “Natural history of placenta previa ascertained by diagnostic ultrasound.“ Am J Obstet Gynecol. 133: 287.

3King DL. (1973). “Placental migration demonstrated by ultrasonography.” Radiology 109:167. 19.

4Benirschke K, Kaufmann P. (1990) The pathology of the human placenta. New York: Springer-Verlag; p 202.

5 Cho, J. Y., Y. H. Lee, et al. (2008). “Difference in migration of placenta according to the location and type of placenta previa.” Journal of Clinical Ultrasound 36(2): 79-84.

6 Magann, E. F., W. E. Roberts, et al. (2002). “Dominant maternal sleep position influences site of placental implantation.” Military Medicine 167(1): 67-69.


UPDATE: Not only did my placenta move by 28 weeks (woohoo!), I revisited the data and re-wrote this post over on Preg U recently. Check it out – Placenta Previa and the Magic of Placental Migration.