Wondering what's going on during the first three months of baby's development? Find out from Kira Weier, Certified Nurse Midwife with Park Nicollet Midwife Services.

Congratulations on your pregnancy! You’re about to embark on an amazing journey like no other. Pregnancy is an exciting time, but it can also be nerve-racking, especially during the first trimester. There’s a lot going on in your baby's development at 12 weeks.

What's up in there?

Kira Weier explains, “When the egg and sperm meet, they form a single cell called a zygote. The zygote travels down the fallopian tube into the uterus, all the while duplicating itself into a tiny ball of cells called a blastocyst. The ball of cells has now separated itself into layers -- the inner layer will become the embryo, and the outer layer will become the membranes that will protect the embryo. When the blastocyst comes into contact with the wall of the uterus, it will start a process called implantation, where it burrows into the lining of the uterus. The placenta starts to form and that is what will nourish the embryo and fetus as it grows into a baby.”

She adds, "Throughout the first trimester, the foundations for all of the body systems are formed -- eyes and ears, skeleton, muscles, nervous system, lungs, heart and circulation, kidneys and reproductive system."

Risk of miscarriage

You may not be able to contain your excitement and choose to share your pregnancy news early on. Many moms decide to wait until the first trimester is over since, according to the American Congress of Obstetricians and Gynecologists, miscarriages happen in about 15-20 percent of pregnancies, most within the first trimester.

Weier says, “This is a very sensitive time in the development process: Toxic substances -- like drugs or alcohol or other things that can impact development -- can have important consequences in terms of birth defects or other problems. Sometimes, when things are not developing correctly, it is due to an outside cause like drugs or other substances. Other times, there is an error when the cells are dividing or multiplying. Whatever the cause, when things are not developing normally in this early stage, sometimes your body will recognize it and the pregnancy will end in a miscarriage. Many early miscarriages, especially in the early first trimester, are due to errors that happen as the cells are dividing and forming the early organ systems.”

Your growing fetus

Hopefully, everything will go smoothly during your first trimester of pregnancy! As Weier explains, "By the end of the first trimester, the baby is about two-inches long and it weighs about half an ounce."

More about the first trimester

Morning sickness until midnight
I'm pregnant! Creative ways to share the news
The five most surprising things about the first trimester