Stage of development of the early embryo that has undergone blastulation, in which a fluid-filled cavity forms in the formerly solid ball of cells (the morula), about 5 days after fertilisation. For the first time, a distinction can be made between a sheet of cells to one side, which will form the embryo proper, termed the inner cell mass, and the remaining, peripheral cells that constitute the trophectoderm, which -- after the blastocyst "hatches" through the zona pellucida and undergoes implantation -- will form the trophoblast. An old-fashioned term for an inevitable miscarriage, meaning that the ovum (in its classical sense for professional embryologists) has not developed normally after fertilisation, there being present just the supporting tissues and no embryo. The term is descriptive, it has no diagnostic value as to the cause of the miscarriage.

The last stage of development of the blastocyst before it hatches out of the zona pellucida prior to implantation.

An in vitro fertilisation manipulation in which a small opening is made in the zona pellucida of the early embryo to help the blastocyst emerge prior to implantation. Formerly carried out by judicious use of an enzyme to dissolve the zona (but risking damage to the embryo), leading IVF programs use a microlaser.

Cells in the developing placenta that are derived from the early embryo's outer layer (the trophectoderm of the blastocyst) and then persist through pregnancy, producing either syncytiotrophoblast or extravillous trophoblast.

A step of major reorganisation of the developing embryo, coinciding time-wise with the missing of the menstrual period in the early pregnant woman, in which the third, or 'middle' layer (the mesoderm) forms. Occurs after blastulation (the transformation giving rise to the blastocyst) and before neurulation (the transformation forming the brain and spinal cord).

The process by which the embryo's trophoblast attaches to the mother's endometrium and penetrates it, establishing contact between the trophoblast's developing chorionic villi and the maternal blood. See also blastocyst. Signifies the commencement of gestation or pregnancy.

Group of cells that differentiates within (but to one side of) the developing embryo when it is a blastocyst and which will, if all goes well after implantation, form the embryo-proper, or fetus. Cells from the inner cell mass can be used to produce embryonic stem cells.

A state in an embryo, fetus, animal or person in which a small number of cells occur that have a different genome. Arises naturally because cells (rarely, stem cells) can cross between a mother and her fetus, or experimentally by introducing foreign stem cells into an embryo, usually at the stage of blastocyst, when the introduced cells become part of the inner cell mass.

Identical or monozygotic twins in which the split has occurred after formation of the blastocyst, so that the twins share the same gestational sac (or amniotic cavity). There is a much higher risk of complications than in the more usual situation where twins do not share the same sac, because the fetuses or the umbilical cords can get tangled up.

A stage of the embryo that consists of a ball of cells, still enclosed by the zona pellucida, before the next stage of blastocyst. Formed from the fertilised egg, or zygote, by the processes of cleavage and compaction.

A new term for the normal cessation of female fertility up to 10 years before the menopause, being apparent in some women after the age of 33 and most women by 45. Different from the perimenopause, which occurs as egg numbers fall far enough to cause shortening of the menstrual cycle, particularly the follicular phase, and accompanied by elevation of serum FSH when measured during menstruation. If pregnancies are attempted through the oopausal transition, a woman who has had no prior reproductive disturbance will typically experience recurrent miscarriages before developing otherwise unexplained infertility, manifesting in the IVF lab sequentially with unexplained implantation failure of apparently satisfactory embryos, then, in turn, a decreased rate of forming blastocysts, defective cleavage, and then failure of fertilisation. There is no known method of overcoming its effects short of egg donation, except for some hope that the procedure of cytoplasmic transfer might be helpful.

A term sometimes used for the embryo or ovum from the stage of fertilised egg (or zygote) up to the stage of the morula. During this time any of the cells of the fertilised ovum can develop into a whole new embryo -- they are totipotent. As it becomes a blastocyst a proportion of the cells (the inner cell mass) commit to producing the embryo proper, while the remaining majority are destined to form trophoblast. Distinguishing pre-embryo from loose use of the term embryo has some value when debating the morality of embryo research, but it is not universal practice.

(PGD) Genetic diagnosis of an IVF embryo before embryo transfer. Made possible by removing one or two cells of the embryo (embryo microbiopsy). Typically performed on day 3 of embryo development in vitro, at the 8-cell stage, prior to compaction, or on day 5, as an expanding blastocyst.

Group of cells that differentiates around the periphery of the developing embryo when it is a blastocyst and which will, if all goes well after implantation, form the placenta and membranes.

The tough but glassy-looking membrane that starts to surround the egg (the primary oocyte) while it's still in the follicle, protects it against sperm which have not undergone the acrosome reaction at its surface, and keeps the cells of the early embryo together until the embryo, as a blastocyst, hatches through it in preparation for implantation.