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Your Baby’s Development: The First Trimester

The moment you find out you’re pregnant, everything changes. There’s a whirlwind of questions and information and to-do lists going on inside your head. You’ll probably start imagining your little one, how they’re developing, what they’ll be like. How big are they now? What about now? When will you feel them kick? When can they hear your voice?

The First Trimester

Fetal development starts very soon after conception, and it usually follows a predictable, steady course. Keep reading to learn more about what your baby’s up to in the first trimester! 

Week One & Two

This might come as a surprise—but you’re not actually pregnant for the first week or two of pregnancy. Conception usually occurs about two weeks after your last period begins. That means, to calculate your estimated due date, your healthcare provider will count ahead 40 weeks from the start of your last period. So, your period is counted as part of your pregnancy—even though you weren’t pregnant at the time. 

Week Three

The sperm and egg come together in one of your fallopian tubes to form one entity called a zygote. If more than one egg is released and fertilized, or if the fertilized egg splits in two, you might have multiple zygotes—twins! Usually, the zygote has 46 chromosomes, 23 from the biological mother, and 23 from the biological father. These chromosomes help determine things like your baby’s sex and physical traits.

Soon after fertilization, the zygote travels down the fallopian tube toward the uterus. At the same time, it will start dividing to form a cluster of cells resembling a tiny raspberry, called a morula. 

Week Four

This ball of cells will start rapidly dividing, and at this point, it’s known as a blastocyst. It starts to burrow into the uterine lining, which is called implantation. Inside the blastocyst, the inner group of cells will become the embryo! The outer layer will give rise to part of the placenta, which will nourish your baby throughout the pregnancy. 

Week Five

This is your fifth week of pregnancy, but remember—it’s only the third week after conception. The levels of HCG hormone produced by the blastocyst quickly increase. This is to signal your ovaries to stop releasing eggs and produce more estrogen and progesterone. 

These increased levels of these hormones stop your menstrual period, which is often the very first sign of pregnancy! It also fuels the growth of the placenta. The embryo also now has three layers:

  • The top layer, the ectoderm, will eventually become your baby’s outermost layer of skin, central and peripheral nervous systems, eyes, and inner ears. 
  • The middle layer, the mesoderm, will become your baby’s heart and primitive circulatory systems. 
  • The innermost layer, the endoderm, will become your baby’s lungs and intestines. 

Week Six

Your baby’s growth will really kick it up a notch this week! Just four weeks after conception, the neural tube along your baby’s neck is almost closed. Their brain and spinal cord will develop from the neural tube. Your baby’s heart and other organs are also starting to form, and their heart will begin to beat. The structures that are necessary to the formation of your baby’s eyes and ears develop at this time, too. Small buds appear that will eventually become your baby’s arms. Your baby is in a C-shape at this point in your pregnancy. 

Week Seven

Seven weeks into your pregnancy, five weeks after conception, your baby’s brain and face are developing. Little sunken areas are visible, that will eventually become their nostrils. Their retinas are just beginning to form. Their legs are following close behind their arms—little buds where their legs will be are visible now. The little arm buds are now resembling paddles. 

Week Eight

Your baby has a nose now, and an upper lip! Their lower limb buds look like paddles at this point, and their fingers are beginning to take shape. Your baby’s ears and eyes are obvious now, and their posture is starting to straighten out. 

By the end of this week, your baby might be about ½ inch long from their crown to their rump—about half the diameter of a United States quarter!

Week Nine

In your ninth week of pregnancy, seven weeks after conception, your baby’s arms grow, and elbows appear. Their toes are beginning to form, as well as their eyelids. Your baby’s head is pretty large at this point, but they don’t quite have a chin yet. 

By the end of this week, your baby is a little less than ¾ inch long—the diameter of a U.S. penny. 

Week Ten

By your tenth week of pregnancy, eight weeks after conception, your baby’s head has gotten much rounder. They can bend their elbows now! Their toes and fingers lose their webbing and start to get longer. Eyelids and external ears are continuing to develop. The umbilical cord is now clearly visible. 

Week Eleven

Your baby’s head makes up about half of its length right now—but don’t worry, their body will soon catch up! Your baby is now officially described as a fetus at this point. Their face is broad at this stage, with their eyes widely separated and their eyelids fused and their ears set low on their head. Buds for their future teeth appear at this point. Red blood cells are also beginning to form in your baby’s liver as well. 

By the end of this week, your baby’s penis or clitoris and labia majora will start developing! Your baby is also around two inches right now—the length of the short side of a credit card. 

Week Twelve

Your baby is getting their fingernails! Their face has now taken on a more developed profile, and their intestines are now developed in their abdomen. Your baby is now about 2 ½ inches—the length of the short side of a U.S. dollar bill. 
Stay tuned for our complete breakdown of the second trimester! Until then, check out our blog for more information.