What to Expect During Your First Trimester

Your first trimester is 13 weeks of new developments and new experiences. Learn how your body changes and how your baby develops week by week. 

Weeks 1-3

During this time the sperm and egg fuse (otherwise known as fertilization) resulting in a zygote, a single-cell organism made up of genetic material from both you and your partner.

Your baby is now known as a blastocyst, a teeny ball of cells, and is busy settling into his or her new home (your uterus), prepping for all the crucial development that will happen over the next six weeks.

Week 4  

Your baby/blastocyst is splitting into two groups this week. One will become your son or daughter while the other becomes the placenta — the lifeline that channels nutrients to your baby and carries waste away.

Your baby is now the size of a poppy seed!

Week 5 

Congratulations – you’re one month pregnant!

At this time most moms are just finding out they are pregnant all the while your baby’s circulatory (or blood) system, along with the heart is starting to form. This is the first part of that tiny body to be functional. Also starting to form is your baby’s neural tube, which will eventually become your baby’s brain and spinal cord.

Your baby is now the size of an apple seed.

Week 6

Your baby’s jaw, cheeks, chin, eyes, ears, and nose are beginning to form what will eventually become one adorable face. In addition, his/her kidneys, liver, and lungs are developing, and their heart is now beating 80 times a minute and getting faster every day.

You baby is now the size of a sweet pea.

Week 7

Your baby is about 10,000 times bigger than he/she was at conception. Most of this growth is concentrated in the head as new brain cells are generated at the rate of 100 per minute. Your baby’s mouth, tongue and little arms and legs are also started to form.

Your baby is now the size of a blueberry.

Week 8

This is a big week as your little one’s lips, nose, eyelids, legs, and back continue to take shape. His/her heart is beating at the incredible rate of 150 times per minute — about twice as fast as yours! And even though you can’t yet feel it, your baby is now making spontaneous movements.

Your baby is now the size of a raspberry

Week 9

You baby is no longer an embryo but is now a fetus! Since last week he/she has grown about an inch and tiny muscles are begining to form. It’s still too soon to feel anything in your belly, but you might be able to hear something at your next checkup.

You baby is now the size of a cherry

Week 10 

Your baby now has small indentations on their legs that will soon be knees and ankles. In addition, your baby’s arms now have elbows that bend. Tiny buds of baby teeth are forming under the gums and his/hers stomach and kidneys are kicking into gear, producing digestive juices and urine.

You baby is now the size of strawberry

Week 11

This has been a busy week for your baby. Now about two inches long, he/she is growing hair follicles, fingernails, and ovaries (if she’s a girl). You baby now has distinct human characteristics, with hands and feet in front of their body, ears nearly in their final shape, open nasal passages on the tip of their nose, and a tongue and palate in the mouth.

Your baby is now the size of a lime

Week 12

The average fetus at 12 weeks is about 2.1 inches long and .49 ounces. His/her fetal digestive system is beginning to practice contraction movements necessary for eating, and their bone marrow is busy making white blood cells to ward off germs once he/she is born.

Now that your baby’s has most of their important organs, his/her main job is to keep on growing!

Your baby is now the size of a plum

Week 13

Tiny bones are beginning to form in your baby’s arms and legs. Also forming is your baby’s vocal cords -the first step toward saying, “I love you, Mommy”.  And because he/she can now move their arms and legs, they may be able to get her thumb into her mouth.

You baby is now the size of a lemon


Congratulations! You made it through your first trimester!  Read our next blog to learn what to expect during your second trimester!


Source: The Bump, Pregnancy Week By Week, First Trimester Weeks, https://www.thebump.com/pregnancy-week-by-week