The placenta is the baby’s organ, not the mother’s. It grows out of the same early embryo as the baby and carries the baby’s genes. It is built where the two bodies meet, though, so a small part of the finished placenta is the mother’s own tissue. And because the placenta’s DNA isn’t always identical to the baby’s, the question matters for anyone relying on a prenatal blood test.
How the Embryo Builds Its Own Placenta
About 4 to 5 days after fertilization, the embryo is a hollow ball of cells with an outer shell and an inner cluster.1PubMed Central. Super-Enhancers in Placental Development and Diseases The outer shell becomes the placenta’s cells, while the inner cluster becomes the baby.2PubMed Central. Two distinct trophectoderm lineage stem cells from human pluripotent stem cells Both come from the same fertilized egg, so the placenta carries the same mix of genes from both parents as the baby does.
That makes the placenta half dad’s. The baby gets half its DNA from each parent, and so does the placenta, which means the mother’s own DNA is only half of what’s in it. Genetically, the organ belongs to the pregnancy, not to her. That is also why her immune system has to tolerate it, since half its genetic material comes from the father.
The father’s genes do much of the work of driving placental growth.3PubMed Central. Paternal epigenetic influences on placental health and their impacts on offspring development and disease The reason is that some genes are switched on depending on which parent they came from, and in the placenta the father’s copies dominate.4PubMed Central. Paternally expressed genes predominate in the placenta Scientists think of this as a tug-of-war that balances how much the mother gives the baby. Father-side genes tend to push growth and nutrient supply, while mother-side genes tend to hold growth in check. The placenta is the main stage for this balance.
The baby’s blood flows through the umbilical cord into the placenta’s branches. Baby and placenta stay one genetic unit until birth, when the placenta is delivered as the afterbirth.
The Small Part That Belongs to the Mother
The placenta’s attached face includes a thin layer of the mother’s womb lining, a small part of the full-term placenta.5PubMed Central. Exploring maternal-fetal interface with in vitro placental and trophoblastic models That layer helps manage her immune system so it tolerates the pregnancy.6PubMed Central. Exploring maternal-fetal interface with in vitro placental and trophoblastic models Inside, the baby’s branching tissue sits bathed in the mother’s blood.7PubMed Central. Exploring maternal-fetal interface with in vitro placental and trophoblastic models
If you look at a delivered placenta, the two sides tell the story. The rough, lobed side was attached to the womb and holds the mother’s tissue. The smooth, shiny side faced the baby and has the cord. Most of the solid tissue is the baby’s, while the blood in it is largely the mother’s.
That blood carries her DNA in her blood cells, not the baby’s. Nutrients and oxygen pass across a thin barrier between her blood and the baby’s blood vessels without the two bloods mixing freely.
Because the baby’s cells grow into the womb lining, there’s no clean line between the two tissues. A sample taken from the placenta can therefore be mixed with the mother’s cells, which labs have to account for when they test it.
Why the Prenatal Blood Test Reads the Placenta
Placental cells constantly wear out and shed fragments of DNA into the mother’s bloodstream.8PubMed Central. Review: Cell-free fetal DNA in the maternal circulation as an indication of placental health and disease This is the DNA that NIPT (noninvasive prenatal testing), the prenatal blood screen for Down syndrome and trisomies 13 and 18, analyzes.9PubMed Central. Review: Cell-free fetal DNA in the maternal circulation as an indication of placental health and disease It’s often called “fetal” DNA even though it mainly comes from the placenta.10PubMed Central. Cell-free placental DNA: What do we really know? So the test is really a reading of the placenta, which usually but not always matches the baby. The pieces are short and float freely, not inside cells, which is why your report may call it “cell-free DNA” or “cfDNA.”
Placental fragments make up a small share of the DNA in your blood. The lab counts fragments from particular chromosomes and checks whether one is over-represented compared with what’s expected. An extra copy of a chromosome in the placenta tips that count slightly, which is how a flag for Down syndrome arises.
A low-risk result is reassuring because placenta and baby usually match. A high-risk result is a screening flag, not a diagnosis. Doctors confirm it with a diagnostic test of the baby’s cells, such as amniocentesis. Because the screen needs only a blood draw from your arm and no needle enters the womb, it’s used as a first step before invasive tests.
When the Placenta’s DNA Doesn’t Match the Baby’s
Sometimes some placental cells carry a chromosome change the baby doesn’t have; your doctor may call this confined placental mosaicism.11PubMed Central. Confined placental mosaicism It can occur with healthy or affected babies and can affect how well a pregnancy goes.12PubMed. Detection of confined placental mosaicism in trisomy 18 conceptions using interphase cytogenetic analysis When unusual chromosome changes were flagged by NIPT, they were found in the placenta after birth in 18 of 31 cases.13PubMed Central. Studies of post-partum placentas provide insights into the origin of structural chromosomal aberrations In 11 of those, the baby’s chromosomes were normal and the change was only in the placenta.14PubMed Central. Studies of post-partum placentas provide insights into the origin of structural chromosomal aberrations
The mix-up starts after the embryo’s cells begin dividing. A mistake in a division can leave some cells with an extra or missing chromosome. If the error occurs in cells that become the placenta, the baby is spared and the error stays in the placenta. A test of placental DNA picks it up even though the baby’s own cells are normal.
A placenta-only change can still matter. Even with a healthy baby, an abnormal placenta may work less well, so doctors may monitor the baby’s growth more closely.
A lost twin can also skew results. If one twin stops developing early, its placental tissue can keep releasing DNA, so a screen can show a problem the surviving baby doesn’t have.15PubMed Central. Discordant NIPT result in a viable trisomy‐21 pregnancy due to prolonged contribution to cfDNA by a demised trisomy‐14 cotwin In one reported case the lost twin’s DNA was still detectable more than 2 weeks later.16PubMed Central. Discordant NIPT result in a viable trisomy‐21 pregnancy due to prolonged contribution to cfDNA by a demised trisomy‐14 cotwin Tell your provider about any early bleeding or a twin seen on an early scan.
Useful questions to ask are whether the result could come from the placenta alone, whether amniocentesis makes sense, and whether a detailed ultrasound would help. Avoid decisions based on the screen alone.
How Long the Baby’s DNA Stays in Your Blood
After birth, placental DNA in the mother’s blood drops by half about every 16 minutes.17PubMed Central. Rapid clearance of fetal DNA from maternal plasma The placenta has been delivered, so no new fragments are being shed, while the body keeps breaking down the loose pieces. In most women it’s undetectable about 2 hours after delivery.18PubMed Central. Rapid clearance of fetal DNA from maternal plasma A new pregnancy’s screen isn’t confused by DNA from a past one.
Baby Cells That Stay in the Mother for Decades
Unlike loose DNA, some of the baby’s whole cells cross into the mother and can live in her bone marrow and organs for decades.19PubMed Central. Fetal microchimerism and maternal health during and after pregnancy The exchange runs both ways, so mother and child each carry a few of the other’s cells.20PubMed. Fetomaternal microchimerism in tissue repair and tumor development The mother’s cells, mostly immune and stem cells, pass to the baby during pregnancy and possibly breastfeeding.21PubMed Central. Maternal-Fetal Microchimerism: Impacts on Offspring’s Immune Development and Transgenerational Immune Memory Transfer They appear to help shape the baby’s developing immune system.22PubMed Central. Maternal-Fetal Microchimerism: Impacts on Offspring’s Immune Development and Transgenerational Immune Memory Transfer Their full role in health, such as healing or disease, is still being studied.23PubMed. Fetomaternal microchimerism in tissue repair and tumor development
These cells are a tiny fraction of the mother’s cells.
The baby’s cells cross at the place where the baby’s tissue meets the mother’s blood. They travel in her bloodstream and settle in various organs. The mother’s cells reach the baby through the same contact, and later through breast milk.
Having these cells is normal after any pregnancy, including a miscarriage. They won’t show up on routine tests and they don’t change your own DNA. A mother may carry cells from every child she has carried.