First Trimester Development

When you imagine a baby in the early stages of development, what do you see? Often, when we think of the early stages of human development in a pregnancy, we may envision a blob of unorganized cells or a formless and undefined mass in the uterus. Or, maybe, we picture a tiny replica of a full-term baby that just needs time to get bigger. While both of these images were typical in earlier times before modern technology, we now know so much more about a baby’s in-utero development thanks to significant medical and scientific advances. 

Human development begins at conception and continues rapidly throughout pregnancy. By the time a woman realizes she’s pregnant – typically around 5 to 6 weeks – the embryo often already has a pulse and the beginnings of a nervous system. At week 5 gestation, the neural tube containing brain and spinal cord tissue begins to develop, and here begins the baby’s first heartbeats! Typically, a heart rate of approximately 110 beats per minute is normal at this stage, and will normally measure between 110 to 170 beats per minute throughout gestation. 

Moving into the sixth week of development, arm and leg buds begin to form, as well as the early structural components of the ears, mouth, and eyes. Circulation begins in the sixth week as the baby needs proper blood supply to continue developing. In the seventh week, bone tissue begins to replace soft cartilage, and will continue to calcify throughout pregnancy to create a strong structure for the body. By the eighth week, the brain continues to develop and the lungs begin to form. Feet and hands are now visible, and digits are becoming distinct. Often, small movements can be seen as early as eight weeks on an ultrasound, as well as limbs and components of the developing brain and spinal cord.

Between the tenth and twelfth week of development, all key body parts and organs are present, though not positioned in their final places. Ultrasounds show more fetal movement and a human profile with nose, lips, and eyes (eyelids are present to protect the developing eyes). We can even see distinct fingers and toes during this stage of pregnancy! While movement won’t be felt at this time of pregnancy, babies can be seen on ultrasound doing things like opening and closing their mouths and fists, moving their arms and legs, and flexing and extending their knees and ankles.

As the baby enters into the second trimester, it can swallow, the kidneys produce urine, the liver produces bile, and all necessary organs, bones, and muscles are present. These various crucial body parts will continue to mature throughout the second and third trimesters, and become functional for sustaining life after birth!

If you are pregnant and would like to see what stage of development your baby is in, book an ultrasound appointment at an Obria clinic near you!


Cleveland Clinic. Fetal Development. Last reviewed by a Cleveland Clinic Medical Professional: 3/3/2023. Accessed 8/21/2023. Available at:


Department of Health. Stages of Fetal Development – First Trimester. Accessed 8/21/2023.

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