Congratulations on your pregnancy! In the months ahead, you’ll likely wonder about how your little one is growing and changing. When will you feel that first flutter? How big is your baby now?
Fetal development follows an amazing, predictable journey. Here’s what happens during each week of the first trimester.
Weeks 1-2: Preparing for Pregnancy
It may seem surprising, but pregnancy technically begins before conception! Your estimated due date is calculated from the start of your last period, even though conception happens about two weeks later.
Week 3: Conception and Fertilization
Once fertilized, the egg forms a one-celled zygote. This tiny new life has chromosomes from each parent, determining your baby’s genetic traits and sex. The zygote divides into a ball of cells and moves towards the uterus.
Week 4: Implantation
The growing bundle of cells, now called a blastocyst, begins to implant into the uterine lining. The inner part will become the embryo, while the outer cells develop into the placenta.
Week 5: Hormonal Changes Begin
By week five, hormones like HCG rise, signaling the ovaries to increase estrogen and progesterone levels, essential for your baby’s development. The embryo now has three layers, each responsible for forming specific organs and body systems.
Week 6: Neural Tube Formation
Rapid growth continues as the neural tube — the early structure for the brain and spinal cord — closes. Structures for eyes and ears begin forming, and tiny buds that will become arms appear.
Week 7: Head and Facial Development
In week seven, the brain and facial features develop, with early signs of nostrils and retinas forming. Limbs continue to grow, taking on more definition.
Week 8: Facial Features and Limbs Form
At eight weeks, fingers start to form, and parts of the ears and eyes become more defined. The upper lip and nose take shape, and the neck and torso begin to straighten.
Week 9: Visible Toes and Elbows
Now with visible toes, elbows, and eyelids, your baby is almost 3/4 inch long. Although the head is still large, your little one is growing steadily!
Week 10: Elbows Bend and Digits Separate
In the 10th week, elbows are more flexible, fingers and toes are defined, and the umbilical cord is visible. The face continues to develop, with external ears forming.
Week 11: Genitals Begin Forming
At week eleven, your baby’s head is still large but more proportional changes are starting. External genitalia start to form into either a penis or clitoris and labia.
Week 12: Fingernails and Facial Development
By twelve weeks, tiny fingernails appear, and the face becomes more developed. Intestines shift into the abdomen as your baby continues to grow, now about 2 ½ inches long and half an ounce in weight. Your baby is now the size of a cake pop!
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Each milestone is unique and beautiful, and soon you’ll begin feeling the first signs of your baby’s presence! Sign up to receive relevant week-by-week emails about you, your baby, and tips for feeling your best during your pregnancy. We’ll deliver these emails straight to your inbox wherever you are at in your pregnancy, as early as 12 weeks. Get these emails today!
