Gestational surrogacy is the more common way to complete surrogacies today. In this
situation, the baby does not get any genetics from a surrogate mother; an embryo
created from the intended parents’ sperm and egg (or using a combination with a
donated gamete) is transferred to the surrogate’s womb, where she carries the baby to
full term.
DNA is passed from parent to child. When a woman carries a child that came from a
donor egg (not of any blood or genetic relation to the woman), due to her blood being
the same blood that pumps through the heart of the fetus inside her, does that fact
change or alter the DNA of the fetus/child to show any blood or genetic relation to the
woman who carried that fetus/child?
A woman who carries someone else’s baby contributes very little, if any, DNA to the
child. This is because the child already has its DNA from its mom and dad. Also, the
baby’s own blood passes through its body, not the blood of its surrogate mom.
However, there is a possibility that some DNA could be transferred from the woman to
the fetus that she is carrying. And vice versa, from the child to the mother.
This exchanged DNA will have almost no effect. At most, it will be a few cells (with
DNA) hiding out amongst the trillions of original cells of the child. Too few cells of the
surrogate mother are passed to the child for them to actually pass on any of her
characteristics or affect the child in any significant way.
Remember, a child’s DNA comes from two places: half from its mother and half from its
father.
But here, we’re talking about changes in DNA that happen during pregnancy, while the
child is developing. So, all of what is discussed is true for both mothers carrying their
own child, as well as for surrogate mothers carrying the child of another woman.
The Placenta: Nature’s Screen
A baby forms from an egg (from the mom) that is fertilized by a sperm (of the dad). The
fertilized egg divides and divides to eventually become the child.
In addition to forming the child, it also forms something called the placenta. The
placenta is attached to the inside of the uterus of the mother. The placenta then
connects to the baby by its umbilical cord. (Your umbilical cord used to be where your
belly button is.)
The placenta is very important because it acts like a filter. The placenta prevents much
of the DNA from passing between the mother and the child.

The placenta serves as an interface between the mother and the developing fetus and
has three main jobs: 1) Attach the fetus to the uterine wall, 2) Provide nutrients to the
fetus, and 3) Allow the fetus to transfer waste products to the mother’s blood.The
placenta allows nutrients that the baby needs to pass from the mother to the baby. The
placenta also carries waste that the child doesn’t need back through to the mother. But
the placenta doesn’t let blood or other cells from the mother or child pass through.
This means that, in theory, no DNA gets through either, since DNA is inside of cells. For
the most part, the surrogate mom’s DNA stays on one side of the barrier and the baby’s
on the other.
So, blood, cells, and DNA don’t pass directly between the mother and child. Just the
important things inside the blood do.
This is a good thing. If too many cells from the mother get inside the child, it could be
dangerous for the child. A lot of cells from the child passing to the mother could be
dangerous for the mother, too. This is why the placenta doesn’t let many through.
Nothing is Perfect
Of course, nothing in life is completely perfect, and this includes the placenta.
Sometimes cells can get through the placenta. This can happen in both
directions—from the mother into the child, and from the child into the mother.
It is a bit more common for DNA from the child to get into the mother. But cells from the
mother can get into the child as well, carrying along the DNA of the mother.
At one time, this was thought to be very rare. However, some recent studies have found
a few cells from the mother pass into the child in around 40 percent of pregnancies.
But just like your screened-in room isn’t full of flies, the baby is not chock-full of DNA
from the woman carrying it. The few cells that sneak through the placenta have the
surrogate mother’s DNA, but that’s it.
If these cells stick around, scientists call it “fetomaternal chimerism.” But, only a few
mother cells usually do.
So, in comparison to the trillions of other cells of the baby, the cells that sneak through
don’t have much of an effect. (Some studies have suggested that they might make
autoimmune diseases more likely, but this research is still at an early stage.)
The bottom line is that once an egg is fertilized, the woman carrying the fertilized egg
does not contribute a significant amount of DNA to the fetus. But she could still affect
how that DNA is eventually used. This can affect the child for the rest of his or her life.
And the effects could even be passed down to grandchildren!
Scientists call these other effects epigenetics.