The placenta is your baby’s lifeline. Its position in the womb can significantly affect the health of the mother and baby during pregnancy and just after birth, too. Here’s what to know about how placental position can impact your pregnancy.
What Is a Placenta?
The human placenta is a truly astonishing organ. It’s the only organ in the body that serves two masters, facilitating the exchange of oxygen and nutrients between the bloodstreams of mother and baby without ever mixing them.
It filters out harmful waste and carbon dioxide from the baby’s bloodstream, and even passes antibodies to the fetus just before birth to jumpstart its immunity. It makes the hormones that sustain the mother’s pregnancy, enables her milk production, and even primes her to care for her baby [*].
Many people's first-trimester symptoms of nausea and fatigue go away once the placenta takes over hormone production in the second trimester.
The placenta is fully formed at about 12 weeks’ gestation. Shaped like a pancake, it’s dense with blood vessels inside structures called villi, red on the side attached to the uterus, grey on the other, and about 10” long and 1” thick. It weighs about a pound at delivery.
Although the human placenta is a transient organ that lives for only 9 months, its effects on the offspring remain for a lifetime. [*]
Where Can the Placenta Be Located?
The placenta develops wherever the fertilized egg implants into the uterine wall. This may be at the back (posterior), front (anterior), sides (lateral), top (fundal), or bottom of the uterus. Researchers don’t know exactly why placentation occurs where it does, but maternal sleep position, blood pressure, and blood type have been shown to have an association.
A 2014 study among 500 healthy women with singleton pregnancy found that maternal blood pressure and sleeping position in early pregnancy may affect placental location (placentation).
Supine (back) sleeping position was more frequent in women with anterior placentation and prone (stomach) sleeping position was more frequent in women with posterior placentation. The women who preferred to sleep in right lateral position were more likely to have lateral placentation, while the women who used to sleep in left lateral position were more likely to have fundal placentation.
Fundal placentation was also associated with women who had higher systolic and diastolic blood pressures [*].
In that study, the locations most frequently observed were:
- 53.2% anterior
- 28.8% posterior
- 10% lateral
- 8% fundal
Another study found an association between blood type and placentation. Posterior placentation had a significant association with A-positive blood, while anterior placentation was common in women with O-positive blood [*].
Placenta Locations Explained
Each location has its own set of potential concerns should a complication arise, but there’s no need to panic. Let’s start with identifying the positions and how they may affect the experience of your pregnancy.
Doctors refer to the various locations (ordered most common to least common) as follows:
The placenta is attached to the front wall of the uterus closest to the mother’s abdomen. Anterior placentation accounts for more than half of all pregnancies.
With the padding of the placenta up against your tummy, it may be harder to feel the baby move. It may also be a little more difficult to perform amniocentesis if needed. The baby may be located with its spine against your spine, which could make for increased back pain and a longer or more painful labor. Women with an anterior placenta may be more inclined to sleep on their backs before their pregnancy. In late term pregnancy, sleeping on your back can pose an increased risk of stillbirth. [*]
While not regarded as a cause for concern, anterior placentation has been associated with a later onset of labor, a higher rate of induction and cesarean section, and postpartum complications. [*]
The placenta is attached to the back wall of the uterus. This is the second most common placenta location. In this location, you may feel the baby’s movements earlier and stronger. One study indicated that posterior placentation increased the risk of pre-term labor. [*]
The placenta is attached to the left or right side of the uterus. This is less common than posterior or anterior implantation. Pregnancies complicated by intrauterine growth restriction (IUGR) are almost four times as likely to occur with lateral placentation vs. anterior or posterior. [*] High lateral implantation has been associated with low Apgar scores, which tests the infant right after birth to see how well the baby tolerated the birth process. [*] Additionally, lateral placental location has been linked with preeclampsia and severe postpartum hemorrhage [*].
The placenta is attached at the top of the uterus. The placenta may not start in this position, but it may move here as the baby grows to make room for a vaginal delivery. The fundal placenta forms the weakest point of the membrane over the cervix and as such, increases the risk of premature membrane rupture. In several studies, the fundal location was more likely to be associated with preterm birth and prematurity [*][*].
The placenta sits low and covers all or part of the cervix. A low-lying placenta can be anterior or posterior in its orientation. When covering the cervix entirely it’s called placenta previa, and it’s a serious condition that requires the baby to be delivered via cesarean to mitigate major bleeding. The mother may be required to be on bedrest from 34 weeks on. Placenta previa is more common among women who are over 35, have had a previous cesarean delivery or placenta previa, smoke or drink alcohol during pregnancy, or are pregnant with more than one child.
Fetal Gender and Placental Location: Is There a Link?
An Iranian study from 2014 sought to determine if placental location was linked to the gender of the fetus. The 200 women studied were early in their pregnancies: 11-13 weeks gestation, with 103 anterior and 97 posterior placental locations. Among the anterior group, 72.8% of them had baby girls. But don’t break out the baby dresses just yet: While these results suggest an association between fetal gender and placental location, there is no definitive study showing causation [*].
In cases where the parents need to know early on the gender of the child due to the possibility of an inherited disease, non-invasive prenatal testing (cffDNA) can test the DNA in the mother’s plasma in the first trimester and determine fetal gender with 96.7% accuracy [*].
How to Tell Where the Placenta Is Located
The position of the placenta can be determined by ultrasound scan as early as 10 weeks’ gestation, where it is seen as a thickened rim of tissue surrounding the womb [*].
But up until about 32 weeks’ gestation, it's common for the placenta to migrate up and away from the cervix as the fetus grows. For instance, about 10-15% of placentas are low-lying at the 18-20 week scan but only 0.5% are still low-lying by full term. A low-lying anterior placenta is more likely to migrate than a low-lying posterior placenta [*].
Most placentas move to the top or side of the uterus by 32 weeks to allow the cervix a clear path for vaginal delivery.
Which Placental Position Is Best for Normal Delivery?
The location of your placenta may change the experience of your pregnancy but know that the placenta can attach virtually anywhere in your uterus and still nourish the baby. In all but low-lying positions, vaginal delivery should be possible.
What matters most in placental implantation is where the lower edge of the placenta extends. If it’s too low in the uterus it can cause bleeding and prevent the descent of the baby’s head during labor, requiring a cesarean section.
That said, studies have shown that placental location can be an important factor in pregnancy, delivery, and infant outcomes.
A 2019 Scandinavian study of nulliparous (first-time) pregnant women revealed that, compared with posterior placental location, fundal and lateral placental locations were associated with a few adverse pregnancy outcomes, the most important being very preterm birth, moderate preterm birth (32-36 weeks of gestation), small-for-gestational-age birth, and manual removal of the placenta in vaginal births.
Additionally, lateral placental location was associated with preeclampsia and severe postpartum hemorrhage [*].
A 2013 study found that anterior placentation is associated with an increased risk of pregnancy-induced hypertension, gestational diabetes mellitus, placental abruption, intrauterine growth retardation, and intrauterine fetal death. Posterior placentation had an association with pre-term labor [*].
While the placenta can nourish from almost anywhere in the uterus, there are instances where an issue with the placenta can endanger the health of the mother and baby. These complications are uncommon but serious:
- Placental insufficiency: Also called placental dysfunction or uteroplacental vascular insufficiency, this occurs when the placenta is damaged or does not develop properly and the fetus does not get enough blood supply. It can lead to low birth weight, premature birth, and birth defects, along with complications for the mother, and affects roughly 8% of all pregnancies [*].
- Placenta previa: The placenta covers the opening of the cervix during the last months of pregnancy, obstructing the path for vaginal delivery and leading to postpartum hemorrhage. Bleeding for any reason in the second half of pregnancy, cramps, or sharp pains are common symptoms of this issue, found in 1 of every 200 pregnancies. Women who have had previous pregnancies and prior cesarean sections are at increased risk.
- Placenta accreta: A life-threatening complication in which the placenta attaches too deeply to the wall of the uterus — sometimes through to the uterine muscle (placenta increta) or into another organ, like the bladder (placenta percreta). It can result in severe bleeding after delivery. Where detected before delivery, this usually results in an early cesarean delivery and removal of the uterus. Occurs in 1 of every 533 pregnancies.
- Placental abruption: A condition where the placenta separates from the uterus too early in the pregnancy, restricting the flow of nutrients to the fetus. 4 out of 5 women will experience vaginal bleeding from this condition, which occurs in 1 in 100 women [*].
- Retained placenta: When part of the placenta stays inside the uterus more than 30 minutes after delivery. There are several types of retained placenta, depending on whether the uterus didn’t contract enough to expel the placenta (placenta adherens); the placenta came away from the uterus but got trapped when the cervix closed too soon (trapped placenta), or the placenta attached too deeply to be expelled (placenta accreta). Infection and blood loss are key symptoms.
When to See a Doctor
With prompt attention, many of the complications associated with an abnormal placenta can be mitigated, though its location cannot be changed in the womb while you’re pregnant. If you experience pain, bleeding, cramping, fast or constant contractions, decreased fetal movement, firmness in the uterus, or abnormal discharge during or after your pregnancy, contact your doctor.
Be sure to take advantage of ultrasound scans regularly as recommended by your OBGYN to head off any potential problems before they become life-threatening to you or your baby.
The Bottom Line
There’s no way to control where your placenta attaches to your uterus, and there’s no changing it after the fact. The best thing you can do is monitor and mitigate in tandem with your care team.
One day, researchers may be able to predict where the placenta will implant in the womb, but until then, mothers and their babies can celebrate how amazingly flexible the placenta is in its positioning inside the womb.
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