Embryonic development begins with conception, when a sperm (male reproductive cell) fuses with an egg (female reproductive cell) in the fallopian tube (a tube connecting the ovary to the uterus), forming a zygote (a single cell with 46 chromosomes, combining genetic instructions from both parents). The zygote undergoes mitosis (cell division into identical cells), forming a morula (a 16–32 cell ball of totipotent cells, capable of becoming any body part) by day 3–4. Genes CCNA, CCNB (cyclin genes regulating division timing), and CDK1 (cyclin-dependent kinase, a protein facilitating division) are activated in the nucleus (cell’s control center), where RNA polymerase II (an enzyme copying DNA to mRNA) produces mRNA (gene instruction messages). Ribosomes (cellular protein factories) translate mRNA into proteins interacting with the centrosome (division organizer) and mitotic spindle (microtubule fibers separating chromosomes). These genes ensure sufficient cell numbers for the baby’s tissues, including the nervous system (brain and nerves), spine (backbone), and brain; errors could halt development. CDK4/6 (division regulators), RB1 (retinoblastoma, preventing excessive division), and E2F (division activators) maintain controlled growth, while MYC enhances CCND (Cyclin D, a division protein) for rapid morula formation. Maternal mRNA and proteins (egg-derived instructions and proteins) initially drive divisions, sustaining the embryo until zygotic genome activation (ZGA) (embryo using its own DNA) at the 8-cell stage, activating OCT4, SOX2, NANOG, and KLF4 (transcription factors, proteins controlling genes). These genes maintain totipotency, enabling cells to form all body parts, with OCT4 preventing early specialization, SOX2 promoting versatility, NANOG preserving potential, and KLF4 reinforcing OCT4 in a feedback loop. TET1/2 (enzymes removing DNA methylation tags) and miR-290 (microRNAs, small gene regulators) ensure precise gene activation. Morula cells adhere via CDH1 (E-cadherin, a cell-binding protein), forming adherens junctions (cell connections) with CTNNB1 (β-catenin, stabilizing connections). By day 4–5, the morula becomes a blastocyst (a fluid-filled structure) with an inner cell mass (ICM) (pluripotent cells forming the embryo) and trophoblast (cells forming part of the placenta, the nutrient-supplying organ). OCT4, SOX2, NANOG, and KLF4 sustain ICM pluripotency, while CDX2 and GATA3, supported by YAP1 and TEAD4, direct trophoblast to placental development, with CDX2 suppressing OCT4 and LATS1/2 preserving ICM potential. The sperm’s DNA integrates with the egg’s, defining traits like hair color. The egg’s protective layer blocks additional sperm. The zygote travels to the uterus over days. The morula’s cells are uniform, poised for specialization. The blastocyst’s cavity aids implantation preparation. The ICM is small but critical for the embryo. The trophoblast prepares to connect to maternal blood vessels. This stage establishes the genetic and cellular foundation for development. No organs exist, only cells ready for transformation. What the baby looks like: No visible baby exists; it’s a microscopic cell cluster, like a tiny berry, invisible without a microscope.