As the cells of the early embryo specialise, they soon differentiate into: (1) central ones that will form the embryo or fetus itself; and (2) peripheral ones (the trophoblast) that will be responsible for invading the mother's tissues (see implantation) and will form the placenta and the membranes.

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.

The outermost layer of trophoblast, produced by cytotrophoblast where there is direct contact with maternal decidua rather than blood. The EVT cells travel into the decidua, reacting with NK cells and invading maternal blood vessels feeding the placenta, softening the walls and replacing the lining with fetal tissue, a process called "conversion". Also known as interstitial or intermediate trophoblast.

The component of the trophoblast bathed by maternal blood in the placenta (and thus covering the chorionic villi). Derived by differentiation and fusion of cytotrophoblast cells, so the syncytiotrophoblast only ever comprises just one cell (albeit with very many cell nuclei), through which almost everything reaching or leaving the embryo or fetus must pass. By the end of normal pregnancy, its area is about 12 square meters, or over 100 square feet. Most hormones and proteins produced by the placenta, such as progesterone, hCG and PAPP-A, come from it.

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.

A gonadotropin produced by the trophoblast of the placenta that acts like luteinising hormone.

The general name for one of the two principal female sex hormones (the other is progesterone), responsible for stimulating growth of the female reproductive system (the vagina, the cervix, the uterus and the fallopian tubes) and growth of the breasts. The main estrogen is estradiol, produced by: the developing follicle (and to a lesser extent by the corpus luteum) in the ovary; by the trophoblast of the placenta; and by the body's fat tissues (through conversion from male sex hormones, or androgens, in the blood). After the menopause the main estrogen is the weaker one, estrone, largely derived from conversion by the body's fat of the weak androgen androstenedione.

(hCG) A gonadotropin produced by the placenta in pregnancy (specifically it's produced by the trophoblast of the chorionic villi); the hormone measured in performing a pregnancy test. The generic (no frills') name for Pregnyl and Profasi, which are preparations of hCG obtained by extracting it from the urine of pregnant women, and Ovidrel, which is made by recombinant gene technology. Mimics the action of luteinising hormone (LH), but has a very much longer duration of action -- and this gives hCG considerable advantages over LH in clinical use. Given as an injection to lead to ovulation from a mature follicle 38 hours after the injection; or to stimulate ongoing function of the corpus luteum, particularly its production of progesterone. So it is typically given after a course of follicle stimulating hormone (FSH) in assisted conception (IVF or GIFT) programs and ovulation induction programs 36 hours before the expected time of egg retrieval (or before having sex or IUI), and then sometimes in further, smaller doses to support the luteal phase that follows. Sometimes used with clomiphene. Ovarian monitoring is needed for its correct use with FSH or with clomiphene. hCG treatment can precipitate the ovarian hyperstimulation syndrome (OHSS).

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.

The space in the placenta in which the maternal blood flows. Washes around the chorionic villi, which contain blood capillaries from the fetus, so that oxygen and nutrition pass to the fetus, while carbon dioxide and waste pass to the mother. Formed from lacunae and lined by the syncytiotrophoblast.

Latin for "lakes": one lacuna, two lacunae. Refers particularly to spaces that form within the syncytiotrophoblast (which therefore continues to line the spaces), separating the SCT into strips of land and little peninsulas that will develop into chorionic villi, as the lakes coalesce into one large intervillous space, into which maternal arteries will open, delivering blood to the placenta as it is being formed.

'Pregnancy-associated plasma protein A': a protein made by the syncytiotrophoblast of the placenta to increase its sensitivity to grow. Found in increasing levels in the pregnant woman's serum as pregnancy develops. Decreased if the pregnancy is affected by Down syndrome, therefore measured in the triple test used for fetal trisomy 21 screening.

That part of the products of conception apart from the fetus where blood from the fetus and from the mother come into intimate contact to exchange nutrients and waste. Composed of trophoblast. Together with the membranes (of the gestational sac) and the umbilical cord composes the 'afterbirth'.

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.

(RTF) A protein produced by the trophoblast that reduces the tendency of the mother's immune system to reject the pregnancy, apparently by increasing levels of interleukin 10, and thus reducing the tendency for miscarriage.

A state of 69 chromosomes, or three times the haploid number, most commonly caused by fertilisation of the egg with two sperm; the embryo that results can develop as far as a fetus, but is doomed to miscarry. The trophoblast of the placenta often undergoes partial changes of a hydatidiform mole. The noun form for this adjective is triploidy (the state of being triploid).