Early embryonic development covers the critical timeline from the moment of fertilization until the embryo successfully implants into the uterine lining. Immediately after a sperm fertilizes an oocyte, the resulting single-celled zygote embarks on a journey down the fallopian tube toward the uterine cavity. During this transit, the embryo undergoes rapid mitotic divisions known as cleavage without increasing in overall size. This remarkable phase transforms a single microscopic cell into a complex multicellular structure ready for survival outside the maternal reproductive tract. Understanding these early milestones helps midwives explain the earliest stages of pregnancy to mothers who are eager to know what is happening inside their bodies during the first two weeks after conception.
Shortly after fertilization, the zygote starts dividing through mitosis. These early cell divisions are called cleavage.
Even though the number of cells multiplies quickly, the total mass remains enclosed inside the zona pellucida, meaning individual blastomeres become progressively smaller with each split.
Around the eight-cell stage, a vital process called compaction takes place. The loosely arranged blastomeres flatten against each other, maximizing cell-to-cell contact and forming tight junctions. This creates a distinct inner and outer polarity within the embryo.
By the fourth day post-fertilization, the embryo consists of 16 to 32 cells and resembles a tiny mulberry. At this stage, it is formally called a morula (from the Latin word for mulberry). The outer cells will soon lay the groundwork for placental structures, while the inner cells are destined to form the actual tissues of the developing fetus.
As the morula enters the uterine cavity on day four or five, fluid begins to seep through the zona pellucida into the intercellular spaces. This fluid accumulates and creates a single large cavity called the blastocystic cavity or blastocoele.
With this fluid shift, the embryo becomes a blastocyst. Two distinct cell populations now emerge:
Before the blastocyst can implant into the endometrium, it must escape from its protective shell. This process is called zona hatching. Enzymatic digestion and expansion cause the zona pellucida to rupture, freeing the blastocyst so its trophoblast cells can make direct contact with the uterine lining.
Meanwhile, the maternal endometrium undergoes essential hormonal changes driven by progesterone from the corpus luteum, entering the secretory phase. The uterine glands become coiled, and the stroma swells with nutrients, creating a welcoming environment known as a decidua ready to receive the incoming blastocyst.