Pregnancy First Trimester: Your Baby's Development

Articles

During the first trimester, your baby grows rapidly along a predictable course. Development begins soon after conception and your child transforms from a small bundle of cells into a fetus, with fully formed eyes, arms and facial features.

Pregnancy First Trimester: Weeks 1 & 2
During the first 2 weeks of your first trimester you aren’t actually pregnant yet. Conception generally occurs 2 weeks after your period starts, so your due date is calculated by counting 40 weeks ahead from the start of your last period.

Pregnancy First Trimester: Week 3 – Fertilization
During the third week fertilization occurs, when the sperm and egg join in the fallopian tube to form a single-celled zygote. If more than one egg is fertilized, you will have more than one zygote. A zygote has 46 total chromosomes, 23 received from both you and your partner. These chromosomes have already determined your child’s hair color, eye color and gender, and even their intelligence to some extent. Shortly after fertilization occurs, the zygote moves down to the uterus. It is already dividing quickly to become a cluster of cells. The inner collection of cells become the embryo, while the outer cells become the membrane that feeds the embryo as it grows.

Pregnancy First Trimester: Week 4 – Implantation
When it reaches the uterus, that small bundle of cells is now considered a blastocyst and is comprised of the embryo and the membranes. When it makes contact, it burrows into the tissues of the uterine wall, which is called implantation. The placenta is also starting to form.

Pregnancy First Trimester: Week 5 – Beginning of the Embryonic Stage
This week of your pregnancy is the start of the embryonic period, when your baby’s brain, heart, spinal cord and organs begin to develop. The embryo is now comprised of 3 layers. The top layer, or the ectoderm, will become your child’s outer layer of skin, as well as the central nervous system, peripheral nervous system, inner ears, eyes and connective tissues. The middle layer, or mesoderm, becomes the child’s heart and circulatory system, as well as the foundation for the kidneys, reproductive system, bones and muscle. Finally, the other layer, known as the endoderm, will turn into a tube that’s lined with mucous membranes. Eventually, the bladder, intestines and lungs will develop in this tube. Your baby is now the size of a pen’s tip.

Pregnancy First Trimester: Week 6 – Closing of the Neural Tube
Your baby’s development is now picking up speed as the neural tube along the back closes and the heart begins to pump blood. The child’s facial features are also starting to develop and the body is getting a C-shaped curve. Arm and leg buds are also starting to appear.

Pregnancy First Trimester: Week 7 – Head Development
Your child’s brain and face are now developing very quickly and small nostrils are visible. The eye lenses are starting to form, while the arm buds are getting paddle shapes. The baby is now the size of a pencil eraser.

Pregnancy First Trimester: Week 8 – Eyes Appear
By the 8th week, the arms and legs of your baby are now getting longer, while fingers are just starting to form. The basic shape of the ears are forming, but the eyes are now visible. The baby’s nose and upper lip have also formed, and the baby’s trunk is straightening out. Your baby is about 1/2 inch long by the end of the 8th week.

Pregnancy First Trimester: Week 9 – Formation of the Toes
During the 9th week of pregnancy, your baby’s arms have grown. The bones are also developing, as well as the bends at the elbows. Toes are forming now as well, and the ears and eyelids continue to develop. By the end of the 9th week, your baby is nearly 3/4 inch long.

Pregnancy First Trimester: Week 10 – Neck Development
Your baby’s head is now rounding out while the neck is starting to develop. The eyes are starting to close as well to protect the eyes as they undergo changes.

Pregnancy First Trimester: Week 11 – Genital Development
Your baby’s head is now nearly half of its entire length. The body is catching up, although this takes many more weeks. The baby is also now officially a fetus. The eyelids are fully fused, the ears are set low and the eyes are wide apart. The baby’s liver is beginning to produce red blood cells. Finally, your child’s external genitalia are starting to form into a penis or a clitoris with labia majora. The baby is now around 2 inches long and weighs about 1/3 ounce, or 8 grams.

Pregnancy First Trimester: Week 12 – Fingernails Form
The 12th week marks the end of your first trimester. Your child has now developed fingernails and the face has a distinct human profile. Your baby is probably around 2-1/2 inches long and weighs nearly 1/2 ounce, or 14 grams.