The word "fetus" is also a medical term for a certain age, or a definite period of time in human development after being an embryo but before being born. After a baby is born he or she is called another medical word "neonate".  These words are intended to specify a certain stage of development of the new human in the womb, to medical professionals, in order to give them a more precise understanding of how old the developing human is.

By day 35-38 after fertilization, the Mullerian ducts for the kidneys are present. The cervical flexure [neck] is present. The elbows are present. The fingers have the joint creases. Toe beginnings are present. Eyelids are forming. The pigmented retina is covered by the sclera [white] of the eyes. The tip of the nose is present. The inner ear has 1-3 of the semi-circular ducts. The secondary bronchi in the lungs are present. The long ureter from the kidney's renal pelvis is branching to form calyces. in the heart the aortic valve and pulmonary valves are present.

By day 38-40, the trunk if elongating, and straightening. The arms and legs are formed. Toes are present but not distinct yet. The arms are longer and bend at elbows. The baby can touch his or her nose with the hands. The temporafrontal region of the brain begins to form.

By day 42-44, the superficial scalp plexus [the network of veins and nerves] has spread,. Fingers are longer and have finger pads at the tips. The hands can bend at the wrist. By day 44-46 the eyelids  almost completely cover the eyes, and the ears are well formed.

By day 46-48 the head is more erect and the neck more distinct. The limbs are longer, more developed.

By week 7 after fertilzation the baby is moving about, and ultrasound can detect the beating heart.

By week 8 after fertilization, the baby is now called a fetus, a medical term for a baby in the womb older than 8 weeks of gestation.