How Can Mother and Baby Have Different Blood Types? The Placental Barrier

Patients ask questions all the time about how the baby gets food and air while it is floating in the amniotic fluid–do mother and baby actually share blood? If not, how does the baby get nourished? If so, how can they have different blood types? Some question  if baby sleeps when Mom sleeps, or gets chilly if Mom is chilly?  Or does the baby know what and when the mother is eating, or if she has heartburn or a headache?

The fact is that the baby’s environment is like a space capsule— the baby lives in a very isolated bag of fluid, connected to the mother ONLY via a network of blood vessels in the placenta. The baby’s world is quite independent of its mother, and the temperature stays constant and warm deep inside the mother’s body at approximately 98.6 degrees. It is completely unaware of the emotions and activities of Mom, aside from being gently rocked about as the mother moves around. The baby naps frequently, having a much shorter sleep/wake cycle than mom and sleeping probably twice as much as mom in a typical 24 hour day. When awake, the baby works on moving, swallowing, and does practice breathing, and after 16 weeks, it can hear some of the louder noises going on ‘outside’ in Mom’s world, including music. There is the constant soothing sound of mother’s beating heart in the background, some digestive noises, and the sound of her voice, but otherwise, baby is only aware of its own small world inside the uterus. Click here for more on baby’s life in the womb.

So how does the baby get nutrients, oxygen, and eliminate waste? It’s all done via the critical organ of the placenta. Mom and baby DO NOT share their  bloodstreams directly and hence they can have completely separate blood types.  The placenta develops early on in the pregnancy and is the lifeline between mother and baby. It is an amazing structure that acts as the lungs AND maidservant, carrying ‘food’ to the fetus and ferrying away the waste, and is comprised mostly of blood vessels. Some of these vessels are the baby’s, and some are the mother’s, but they do not actually connect. The following image demonstrates how this works:


picture showing how mom's bloodstream is separated from baby's blood in the placenta

Image of how mom's bloodstream is separated from baby's blood in the placenta

The mother’s blood vessels extend from the uterine wall into the placenta,in this image extending from the right side to the left. They lie directly next to the fetal blood vessels that grow and extend from umbilical cord on the fetal side of the placenta (seen extending left to right). So, as you can see, the maternal vessels and the fetal vessels do not actually connect, but are nestled very close together. Now, think back to 8th grade biology, and remember those lectures on membranous cell transport mechanisms, i.e.  osmosis and diffusion. Not that we want to get bogged down in technical biology terms here—simply put, the molecules in mom’s blood stream squeeze out of the cell membranes lining the blood vessels and travel into the fetal umbilical vessels and into the baby via the umbilical cord, and vice versa.

Because all of the nutrients and oxygen are carried to the baby by the mother’s blood/circulatory system, anything that affects her blood flow can affect the baby. Click for more information on how smoking, drugs, or hypertension can affect the fetus.

So, back to how food and oxygen get to the baby. Mom’s lungs oxygenate her own bloodstream as she is breathing, and that oxygenated blood passes into the maternal vessels in the placenta. These lie near enough to the fetal blood vessels to allow the oxygen to scoot across the placenta from mother’s vessels into the fetal vessels. Richly oxygenated blood is carried into the baby through the cord, and the excess carbon dioxide travels from the baby’s side over to the mom’s bloodstream for her to process and remove when she exhales. Similarly, when mom eats and drinks, her stomach and intestines do the digesting; processing and breaking down the contents into molecular form to be absorbed and passed into her bloodstream. As her blood circulates through her body, it courses through the uterine vessels and placenta carrying ‘food’ in the form of broken down  molecules of water, electrolytes, glucose, lipids (fats), water soluble vitamins, peptides and amino acids which become proteins in the baby, etc., all of which travel across the vessel walls into the umbilical/fetal vessels and eventually into the developing baby. So the baby, in essence, is not really getting mom’s favorite spicy thai chicken for dinner, nor her ice cream for dessert. Rather, it is being fed ‘ intravenously’ hours after her meal, once her body has broken dinner down into its respective molecular parts and has been absorbed into her bloodstream.

Meanwhile, as the baby processes the nutrients, waste products are created such as urea, ureic acid, and creatinine. These travel from the fetal side of the vessels into the mother’s circulation where she eliminates them from her blood via her own kidneys, liver, etc. The baby does ‘pee’ in the womb, which comprises the bulk of amniotic fluid, but it is not urine in the sense that it serves as waste disposal, rather, it is more a sterile, recycled fluid that the baby drinks, processes, and expels. The actual waste products travel out through the cord, into the placenta, for the mother to process and eliminate. Click for more about amniotic fluid.

The size of the molecules do matter- things need to be small enough to travel back and forth through the cell membranes in order to be passed to and from the fetus. This is what is meant by the “placental barrier.” Alcohol molecules and many drugs are easily transported across cell membranes while larger molecules like large-chain proteins and some infectious microbes are blocked. It is safe to assume that almost everything the mother ingests and processes into her bloodstream is potentially going to be available to the baby in its digested or processed form. However, as with everything, common sense and moderation is key! Being exposed to a rare, few breaths of second-hand cigarette smoke, taking an occasional dose of approved medications such as Tylenol or antibiotics, or even a daily cup of coffee (to name a few examples of things that we hear our patients worry about)  is not likely to pose significant or harmful exposure of toxins to your baby. When in doubt, speak to your doctor or midwife.


  1. Katherine says:

    Thank you so, so, much for this concise and clear explanation of how the mother/child (in utero) circulatory system and placenta ‘barrier’ works!

    You provided an answer, in one glorious(!) webPAGE, for my question that the 4+ webSITES which preceded your listing in my Google search results had failed to answer completely…even after a thorough reading of them all..:-)

    Phew…thank you!

    Now I’m going to spend a couple hours making sure your site now pulls up FIRST in search results to show my appreciation (and help other expectant mothers / knowledge-seekers find you guys FIRST!)

    Thanks again.

    • Thanks so much for your complimentary feedback! I have worked hard on the site and often wonder if I am talking into the wind. It’s nice to know i was able to get you info you were looking for. Please feel free to suggest any other topics as I am always looking for things I should write about. Best wishes for a safe and happy pregnancy!

Speak Your Mind


Copyright © · Prenatal Answers · Advertise With Us