Baby's Blood Type

by Pregnancy.org Staff

drop of bloodDuring the first few days your sweet newborn will undergo a series of tests. These can determine many things from genetic disorders to blood sugar levels and your baby's blood type. Sometimes it's determined at birth from the cord blood and other times when the newborn screen is performed. While you might not put this information on a birth announcement, it's among the most vital things to remember about your child!

Blood Type

Your baby receives genetic information at conception. Each parent donates an A, B or O allele, which is one of two or more forms of a gene. Gene dominance is like cooking with spices. Some are more aggressive. Your dish might contain equal amounts of garlic and paprika, but you'll only notice the garlic. Back to your baby's blood -- A and B are both dominant over O, which means a child who receives an A blood type allele from the father and an O allele from the mother will have an A blood type.

Paternity

If you're wondering who's your baby's daddy, a paternity test can be taken. Because DNA testing is expensive, many couples choose to first compare the baby's blood type to mom's and dad's. Keep in mind that the blood test results aren't able to pinpoint who the exact father is but they can tell who isn't the biological "dad." Here are two examples. In each, both mom and "dad" have blood type O.

  1. The baby's blood type is A, B, or AB. Just like two blue-eyed parents can only have a blue-eyed child, parents with O blood type can only have a baby with blood type O type. The presumed dad is not the real dad.
  2. The baby's blood type is O. In this case the presumed dad might be the real father. But so might millions of other men -- any man with blood type O.

In the second case, the couple may decide to follow up with a DNA test. The table below lists parent's blood types and the possible and impossible combinations for their baby:

baby's possible blood types

There are over 600 known blood types which makes paternity testing far more accurate today than in 1940 when only 4 blood types had been discovered. DNA testing is legally accepted to determine paternity. If the test says that a man isn't the father, then legally he isn't. If the test says that he's the father, there's about a 99.8% chance that he is.

Rh Factor

Just like there are different major blood groups, such as type A and type B blood, there's also an Rh factor group. Each parent carries a set of alleles -- either two +'s, 2 -'s or one of each. Your baby will inherit one from each parent. If either allele is positive, then your baby will be Rh positive.

possible Rh types

The highlighted line shows that two parents who are Rh+ could have an Rh negative child.

Knowing your baby's blood type might not seem important right now, but having this knowledge can help in many situations -- from childhood emergencies all the way to parenthood!

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