Find out What Happens to the Fetus in the First Trimester

A mother's week-by-week guide to fetal development

Woman getting an ultrasound before her mock embryo transfer
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The first trimester starts on the first day of your last period and lasts until the end of week 12. From the moment the fertilized egg implants in the womb right through to the formation of bones, muscles, and organs, a lot will happen during these first three months.

Here is a week-to-week breakdown of what to expect as your baby starts to develop:

Week One to Four

If your periods are regular, you will likely conceive sometime between week one and four.

While the fertilized egg may implant earlier than four weeks, if the first day of your last period was four weeks ago, you are officially four weeks pregnant.

By the time this occurs, the cells of the egg will have begun to rapidly divide and start taking on specialized characteristics. The baby's nervous system and heart will begin to form, and an embryonic circulatory system will eventually become the umbilical cord.

By the end of the first week, a gestational sac will have already formed, and there will be a yolk sac in place to feed the embryo until the placenta is fully developed. The baby will be roughly two millimeters in size.

Week Five

By week five, the baby's face, skull, and brain will start to take shape, and the hands and feet will appear as tiny buds. The outer layer of the amniotic sac, meanwhile, will start to develop into the placenta to supply the baby with oxygen and nutrients.

The embryo will now be around nine millimeters in size.

Week Six

Your baby's heart will be more developed by this stage, and a tiny heartbeat may even be heard on a transvaginal ultrasound. By week six, the baby will be covered with a translucent skin and look rather like a tadpole with a tail and tiny dimples that will eventually become eyes.

Week Seven

By week seven, the brain will have rapidly developed and be larger than the rest of the body. Cartilage will start to form, and a delicate network of nerves will begin to traverse the baby's tiny body. The baby will now be roughly the size of a grape.

Week Eight

The baby will double in size during week eight. At this point, the baby will be gradually weaned off the yolk sac and slowly start receiving nutrition from the placenta. The baby will no longer be classified as an embryo but rather be referred to as a fetus.

Week Nine

By week nine, the baby's eyes will now have eyelids, and the genitals will begin to develop into either a penis or clitoris. The fetus will now be just over two centimeters in size (or roughly an inch).

Week 10

By week 10, evidence of tiny teeth will be seen on the jaw, while the features of the face (including the nostrils and upper lip) will start to take shape. Even the eyes will begin to react when exposed to light. The fetus will have grown to around three centimeters in size.

Week 11

Fingers and toes will be clearly defined and even have tiny fingernails. The internal organs will start functioning to some degree (including the stomach and kidneys). The translucency of the skin will have become more opaque, and the lids covering the eyes will be fully closed.

Week 12

By week 12 and the end of the first trimester, the fetus will be fully formed and roughly the size of an apricot. The placenta will be fully developed, and the cartilage skeleton will have begun to calcify and form into bones. Everything is now pretty much in place—limbs, muscles, organs, and genitals—to take us into the second trimester.