Title: Fetal development timeline | BabyCenter
Headings: Fetal development timeline
Fetal development timeline
IN THIS ARTICLE
First trimester
Second trimester
Third trimester
Karisa Ding

Content: Fetal development timeline | BabyCenter
Pregnancy
Your Baby
Fetal development timeline
By Karisa Ding
|
Medically reviewed by Aaron Deutsch, M.D., maternal-fetal medicine
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IN THIS ARTICLE
First trimester
Second trimester
Third trimester
The transformation from a tiny fertilized egg to a fully formed baby is awe-inspiring. Here's a look at the major milestones babies in utero typically reach, from conception to birth, and approximately when they happen. First trimester
3 weeks
Your baby-in-the-making is a ball of cells called a blastocyst. The blastocyst already contains a full set of DNA from you and your partner, which determines sex, eye color, and other traits. 4 weeks
The ball of cells has officially become an embryo and is about the size of a poppy seed. Over the next six weeks, all of your baby's organs will begin to develop, and some will start to function. 5 weeks
Your baby's tiny heart begins to beat – at twice the rate of yours. His entire "body" is only about the size of a sesame seed. 6 weeks
Facial features (like eyes and nostrils) are beginning to form, and little buds appear where arms and legs will develop. 8 weeks
Arms and legs are growing, and your baby now has little fingers, as well as a nose and upper lip.