First Trimester Fetal Development

During the first trimester, when you’re dealing with sore breasts, nausea, fatigue, and constipation, it may be difficult to pay attention to the baby growing inside you. Because you’re not showing or able to feel any fetal movement yet, your early pregnancy is defined more by maternal symptoms than by the baby’s growth and activity. Yet so much fetal development is happening during those first 13 weeks that it seems unfair to pass them by without proper acknowledgement.

Below is an overview of some of the (amazing!) fetal development goes on while you’re dealing with the side effects of pregnancy.

The weeks are based on standard gestational age, which begins with the first day of the last menstrual period:

Weeks 1 through 4:
At approximately week 3, the pregnancy journey begins as the egg is fertilized to form a zygote that will turn into an embryo. Before the first month is over, embryonic cells have already begun to differentiate so they can support functions involving the kidneys, the nervous system, the heart, and others. A yolk sac is nourishing the embryo until the placenta takes over.

Weeks 5 through 9:
The embryo now has a primitive brain, a beating heart, and arms and legs with webbed fingers and toes! Eye lenses and eyelids are developing. The placenta continues to grow while providing nutrition for the baby and removing waste. By week 9, the baby grows to approximately ¾ inch in length.

Weeks 10 through 13:
During this time period, the embryo changes into a fetus. Facial features emerge, and the neck begins to form. External genitalia is differentiating as female or male. Limbs grow in length, and the wrists, knees, and elbows become more apparent. Fingernails and toenails are developing. Twenty tooth buds appear and the bony skeleton begins to form. The fetus can now produce urine and breathe in amniotic fluid. By the end of week 13, the baby is about 3 inches long.

For some, it’s hard to imagine that a fetus the size of a plum could have so much definition already. Important fetal development occurs during the first trimester, the most vulnerable time for the growing baby, which is why it is so important to take proper care of yourself. Take a moment each day to focus on your baby and feel proud of all the hard work your body is accomplishing. You may not be feeling great, due to your pregnancy symptoms, but at the very least, you can feel good that you are doing something truly amazing.

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