The first trimester sets the foundation for every major system in the human body. The zygote progresses rapidly from a single fertilized cell into a structurally complete fetus. By the end of the embryonic stage at eight weeks, all major organ systems are present, though still immature. The human is then termed a fetus, marking the beginning of the fetal period. By the end of the first trimester, the fetus has a distinctly human form and a greatly reduced risk of major structural developmental issues.

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Week 1: Fertilization

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Fertilization occurs when sperm penetrates the egg, forming a new single-celled human organism called a zygote. This cell contains a complete set of genetic instructions (DNA) that determine sex, eye color, and many inherited traits. The zygote begins rapid mitotic division as it travels through the fallopian tube. Despite dividing, the total size remains unchanged because the cells divide within the original zona pellucida. At this stage, there is no physical connection to the mother. Nutrients come from materials stored within the egg.

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Size: microscopic (single cell)

 

Week 2: Implantation

 

The zygote develops into a blastocyst composed of an inner cell mass and an outer trophoblast layer. The blastocyst implants into the uterine lining. The trophoblast begins forming the placenta, which will mediate nutrient and gas exchange. Human chorionic gonadotropin (hCG) production begins, preventing uterine shedding. The amniotic cavity and yolk sac start forming. These early structures provide protection and nourishment. Maternal blood vessels begin remodeling to support placental circulation.

 

Size: 0.1-0.2 mm

 

Week 3

 

Gastrulation reorganizes the embryo into three germ layers: ectoderm, mesoderm, and endoderm. Each layer will give rise to specific tissues and organs. The neural plate forms and begins folding into the neural tube, which will become the brain and spinal cord. The primitive heart tube and early blood vessels emerge. The placenta grows rapidly, anchoring the embryo securely. This is a critical period of development when environmental exposures can have major effects.

 

Size: 1-2 mm

 

Week 4: Heartbeat

 

The embryo undergoes profound morphological and organ-forming changes as it transitions from a flat disc into a C-shaped body with early organ systems beginning to form. The heart begins beating and circulating blood through primitive vessels. Limb buds emerge, along with early eye and ear structures, and small buds appear that will become arms and legs. The brain rapidly develops as the neural tube closes, establishing the basic structure of the central nervous system. The foundations of the digestive tract and respiratory system are laid.

 

Size: 4-5 mm (poppy seed)

 

Week 5

 

The brain expands rapidly, forming distinct regions such as the forebrain, midbrain, and hindbrain. Facial structures begin shaping the eyes, nose, and mouth. The heart continues strengthening and circulates blood more efficiently. Limb buds elongate, showing early differentiation of arms and legs. The spinal cord extends the length of the embryo. Blood flow supports rapid tissue growth.

 

Size: 7-9 mm (sesame seed)

 

Week 6

 

Arms and legs become more defined, with paddle-shaped hands and feet. Eye pigment appears, and external ear ridges form. The heart develops chambers and beats rhythmically. Early intestines grow rapidly and temporarily extend into the umbilical cord. Neural connections multiply, the neural network starts to fire, and electrical activity begins in the brain. The spine becomes more defined. Movement begins at this stage, although the mother cannot feel it yet. The child has reflexive responses as its nervous system grows.

 

Size: 12 mm (lentil)

 

Week 7

 

Fingers and toes begin separating as webbing recedes. Major organs continue rapid development, including the liver, kidneys, and lungs. The heart has developed four chambers. Facial features become more recognizable, with eyes positioned laterally. Bone development begins as cartilage slowly ossifies. The intestines continue developing. The brain continues to rapidly expand and muscle tissue forms allowing spontaneous movement to begin.

 

Size: 18 mm (blueberry)

 

Week 8: The Embryo Becomes a Fetus

 

All major organ systems are present, although still immature. Facial features such as eyelids, lips, and nose are more defined. The tail-like structure seen earlier regresses. By the end of week 8, the embryo is officially classified as a fetus. The fetus exhibits spontaneous movements and reflexive responses. Brain growth remains rapid and dominant.

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Size: 25 mm (raspberry)

 

Week 9

 

The fetus begins more coordinated movements of the arms and legs as the nervous system continues development and muscles and nerves integrate further. The head remains disproportionately large due to brain growth. Eyelids fuse temporarily to protect developing eyes. The placenta assumes greater responsibility for nutrient exchange. External genital structures begin forming but are not yet distinguishable. The heart now beats ~170 times per minute.

 

Size: 2.3 cm (grape)

 

Week 10: End of the First Trimester

 

Vital organs begin functioning at a basic level, including kidneys and liver. Fingers and toes are fully separated, and nails start forming. The face appears distinctly human with more balanced proportions. Joints allow bending of arms and legs. The fetus can stretch, kick, and rotate slightly. Tooth buds form beneath the gums.

 

Size: 3.1 cm (kumquat)​

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