In his book Christian Ethics, Norm Geisler provides an excellent summary of the known facts surrounding early fetal development thanks to modern fetology:
First Month: Actualization
Conception: All her human characteristics are present.
She implants or “nests” in her mother’s uterus (one week or so later).
Her heart muscle pulsates (three weeks).
Her head, arms, and legs begin to appear.
Second Month: Development
Her brain waves can be detected (40 to 42 days).
Her nose, eyes, ears, and toes appear.
Her heart beats and blood (her own type) flows.
Her skeleton develops.
She has her own unique fingerprints.
She is sensitive to touch on her lips and has reflexes.
All her bodily systems are present and functioning.
Third Month: Movement
She swallows, squints, and swims.
She grasps with her hands and moves her tongue.
She can even suck her thumb.
She can feel organic pain (8–13 weeks).
Fourth Month: Growth
Her weight increases six times (to one-half birth weight).
She grows up to eight to ten inches long.
She can hear her mother’s voice.
Fifth Month: Viability
Her skin, hair, and nails develop.
She dreams (REM sleep).
She can cry (if air is present).
She can live outside the womb.
She is only halfway to her scheduled birth date.