Carnegie Stage 4 Blastocyst begins to Implant - 5 to 6 days post ovulationCarnegie Stage 2 Morula - 4 days post ovulationCarnegie Stage 3 Early Blastocyst Formation - 5 to 6 days post ovulationOocyte Fertilized - 1 day post ovulation
What and Where is the Endometrium?
The uterus is lined with a spongy layer of cells that thickens in preparation for the implantation of a fertilized egg. No implantation and the endometrium sluffs off, menustration.
Egg being fertilized -
Fertilization of the human egg (oocyte) takes about
24 hours. Eleven hours following fertilization,
a polar body with all excess chromosomes is shed.

Image of human egg being fertilized from the British Broadcasting Corporation - Education, BBC

The Morula -
a solid ball of about 12 to 32 cells. It forms around day 3 after ferilization.

Possible time for Pre-Implantation Genetic Diagnosis (PGD)
The Blastocyst -
forms after a fluid filled sac (a blastocycle) appears in the center of the morula, The embryo now has TWO cell types.

Embryonic Stem Cell lines may be generated at this point.

Ovulation - The egg erupting from the ovary. The Visible Embryo dates from this point in time for all of pregnancy.
Embryo Implants in Uterine Wall

Blastocyst "hatches" from the zona about the sixth day after fertilization.

As it enters the uterus, the outer layer of trophoblast cells secrete an enzyme to erode the lining of the uterus, allowing the blastocyst to implant.

Human chorionic gonadotropin (hCG) secreted by the trophoblast cells begin to spike.
The Visible EmbryoHome
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Ovulation Occurs in Two Phases

The first phase of the ovulation cycle is the follicular phase. It begins the first day of the last menstrual period (LMP) and stops at the next ovulation. This phase varies greatly for each woman.

The second phase is the luteal phase. It begins the day of ovulation and lasts until the next menstrual cycle begins. Governed by hormone release, it follows a more regular timeline of between 12 to 16 days following ovulation.

The day of ovulation determines the length of a menstrual cycle. The common belief that stress can affect the timing of a period is half true. Stress can affect ovulation which then affects hormone release which begins the next menstrual period ; but, stress around the time of an expected period will not make it late - that date was decided 12-16 days earlier.

What Is Going On Here?

Fertilization begins in the fallopian tube. An egg is fertilized and divides into a morula or "mulberry like" cluster or cells and then into a blastocyst - where the first cavity appears pushing cells to either the outside or inside. As the blastocyst falls, floating through the fallopian tube, about 70 - 75% will implant. Only upon implanting in the uterine wall will 58% have a chance of continuing on to a normal pregnancy.

Where are the Stem Cells?

The blastocyst embryo with its two cell types: the inner cells that can initiate all cells of the body, and the outer cells destined to become the placenta, is the source of embryonic stem cells (esc).

Embryonic Stem Cells (esc) have the potential to become any type of human body cell. Stem cells are the only cells of the body which divide into two unique entities. Typically when cells divide they produce an identical cell or "sister" cell. But stem cells make a new stem cell AND a new body type cell. This property makes them extremely unique.

Blastocysts only become available for use as stem cells when donated by couples using in-vitro fertilization (IVF) techniques. A woman in an IVF program who has multiple eggs fertilized in a petri dish, can choose to donate extra blastocysts to research. These stem cells are useful to children and adults needing interventions beyond conventional medicine.
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When stem cells divide, one becomes a specialised cell,
while the other remains a stem cell. Specialised cells are
all the different cells in our body that perform a function.
They are all the cells we see on us and in us,
such as lung cells, skin cells, hair cells, and kidney cells."

Roger Pedersen,Ph.D.
Professor of Regenerative Medicine,
Embryonic Stem Cells And Stem Cell Technology,
The Naked Scientists Interviews