The outermost of the two membranes surrounding the gestational sac that encloses the developing embryo. Separate from the amnion until about 14 weeks of pregnancy, after which the two membranes stick together without intervening fluid. Like the placenta, with which at the margins it is joined seamlessly, derived from trophectoderm.

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

A tongue of tissue within the placenta composed of chorion and containing capillaries connected to the blood circulation in the fetus that laps the mother's blood in the uterus, exchanging oxygen, nutrients and waste products between the fetus's blood vessels (in the villi) and the mother's blood. This tissue is sampled for genetic testing with a CVS (or chorionic villus sampling. See also intervillous space. Plural: chorionic villus.

(CVS) A test done at about 9 weeks pregnancy at which, under the guidance of transvaginal ultrasound, a catheter is passed through the cervix of the pregnant uterus to obtain a small sample of tissue from the placenta (the afterbirth) for genetic testing, such as a karyotype.

(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 membrane and cavity that encloses the developing embryo and fetus in fluid (essential for all vertebrates that reproduce out of the sea), cushioning the embryo and fetus from the effects of pressure. Unlike the chorion, contains cells that originate from the fetus itself.

Cavity enclosed by the amnion, which grows from the embryo within the gestational sac, gradually catching up with it in size by about 14 weeks, when the amnion and chorion fuse, and after which amniocentesis becomes practicable.

A protein molecule with anticoagulant properties bound to chorionic villi, responsible for assisting maternal blood to circulate in the placenta.

Generic name for recombinant human chorionic gonadotropin.

A drug that blocks the action of estrogens and so tricks the pituitary gland into thinking the ovary's follicles are not producing enough estradiol, so that natural FSH production is temporarily increased, the ovaries thereby are stimulated, and follicles grow. The LH surge and ovulation usually follow naturally, but human chorionic gonadotropin can be given if monitoring is used to make sure the follicle is properly mature. Brand names: Clomid (Marion Merrell) and Serophene (Serono)

(CGH) A molecular DNA diagnostic technique whereby a set of chromosomes (a genome) is compared with a standard set using different colored dyes (typically red and green), so that any areas that do not exactly match will appear green or red instead of brown; a sensitive technique that can be applied to preimplantation genetic diagnosis as well as to chorionic villous sampling, or CVS, and to amniocentesis for prenatal diagnosis, yielding results much faster than is possible with a formal karyotype and more comprehensively than is possible with fluorescent in situ hybridisation.

Latin for yellow body, the description being that of the solid or cystic structure in the ovary after ovulation. Derived from the ovulating Graafian follicle. At first red and friable as arteries and veins invade the collapsed follicle, it soon matures into a gland that is very efficient at producing progesterone, a hormone that's soluble in the fat and which therefore gives the corpus luteum its yellow color. Provides its name to the second, or luteal phase of the ovarian cycle, as well as to luteinising hormone, which causes the corpus luteum to be formed and sustains it until, in the event of pregnancy, it is supported instead by human chorionic gonadotropin. The appearance of the corpus luteum on transvaginal ultrasound is so variable (and so able to be confused with a serious abnormality of the ovary) that for accurate diagnosis of such pathology it is best to scan in the follicular phase or while taking a progestogen to prevent formation of this structure. Plural: corpora lutea.

Due to trisomy 21. Chromosome 21 is the smallest of the "autosomes" (the non-sex chromosomes): trisomies of the other autosomes tend to be lethal at an earlier stage of embryonic or fetal development, and so are seen much more rarely. Diagnosis of Down's syndrome requires a karyotype, obtainable from pregnancy tissue by chorionic villus sampling (CVS) or amniocentesis. Screening for increased risk in pregnancy can be performed by triple screen or by looking for nuchal translucency at transvaginal ultrasound.

Serial transvaginal ultrasounds of the ovary during the follicular phase to track the growth of one or more tertiary follicles, accompanied usually by serial estimations of serum estradiol, serum LH and serum progesterone, for the two-fold purpose of estimating follicular maturity (to time intercourse, a mid-cycle-dependent investigation such as a postcoital test, or an egg retrieval) and identifying the onset of the LH-surge, which can affect the timing of the intervention. Sometimes accompanied by injection of human chorionic gonadotropin to trigger ovulation at a precisely anticipated time.

Any hormone that switches on the function of the gonads. There are two main families of gonadotropins: (a) the gonadotropin that stimulates the growth of the follicle, or follicle stimulating hormone (FSH); and (b) those that cause ovulation from the mature follicle and stimulate the corpus luteum that results to develop and to produce progesterone, namely luteinising hormone (LH) and human chorionic gonadotropin (hCG). FSH will cause growing follicles to produce the estrogen estradiol, provided that a small amount of LH (or hCG) is present. FSH and LH are produced in the pituitary gland, whereas hCG comes from the placenta in pregnancy. In men, FSH stimulates the Sertoli cells of the testicular tubules, and hence drives spermatogenesis; LH and hCG stimulate the Leydig cells to produce testosterone.

Synonym for preovulatory follicle. A large, mature tertiary follicle that will respond to an adequate LH surge or injection of human chorionic gonadotropin (hCG) by undergoing ovulation, releasing its egg. Produces estradiol and, with exposure to LH or hCG, progesterone. Named after Reijnier de Graaf (1641-1673), the first person to see and to appreciate the importance of the ovarian follicle.

An abnormal conceptus in which the chorionic villi of the placenta have become cystic and swollen, typically because the chorion has not developed in connection with a fetus that has a properly functioning circulation. The placenta looks like a bunch of grapes.

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.

A sudden and huge increase in production of luteinising hormone by the pituitary gland in response to sustained and substantial levels of estradiol in the blood in women, midway through the ovarian cycle. Causes ovulation about 36 hours after it starts (about 20 hours after its peak). If not suppressed during assisted conception (see GnRH-agonists and GnRH-antagonists) it can start before human chorionic gonadotropin has been given, so spoiling the timing of egg retrieval. Timely production of the LH surge in adult women depends on female conditioning of the hypothalamus and/or the pituitary gland before birth. Detectable by testing the urine with a special kit.

The part of the ovarian cycle between ovulation and the start of a new follicular phase, so dominated by the presence of the corpus luteum and the progesterone it produces. Normally between 11 and 16 days in length. Extended by the action on the corpus luteum of human chorionic gonadotropin (hCG) if implantation of the embryo is successful. Shorter cycles can interfere with implantation (a luteal phase defect). Corresponds with the secretory phase of the menstrual cycle.

(LH) The hormone, or gonadotropin, produced by the pituitary gland to cause, in women, ovulation of a mature follicle in the ovary, resulting in formation of the corpus luteum; in men, stimulates the production of testosterone by the Leydig cells of the testis. Suppressed by GnRH-agonists and GnRH-antagonists, so that (for ovulation induction) an injection of human chorionic gonadotropin (hCG), which acts the same way, has a predictable time course of action. A small amount of LH is essential for normal function of the tertiary follicle and the corpus luteum. Recombinant LH is under development by Serono as Luveris, but its clinical role in place of hCG is not yet established.

The amniotic membrane and the chorionic membrane attached to the placenta.

For our purpose in the book, the loss of an early embryo at or before the expected time of a period. Not noticeable without specially measuring levels of serum human chorionic gonadotropin One form of so-called biochemical pregnancy.

Recombinant human chorionic gonadotropin made by Serono. Called Ovitrelle in Europe.

The use of drugs to stimulate the development of follicles in the ovaries to undergo ovulation, such as clomiphene, various preparations containing follicle stimulating hormone (FSH), and human chorionic gonadotropin (hCG). The two main situations for it are: in the treatment of infertility due to anovulation typically when there is oligomenorrhea or amenorrhea; and for superovulation in assisted conception (e.g. in vitro fertilisation and gamete intrafallopian transfer)

Nowadays a measurement of human chorionic gonadotropin in serum or urine, usually as a simple 'yes' or 'no' test. In principle a single test cannot distinguish a normal pregnancy from an ectopic pregnancy or one destined to miscarry (a miscarriage).

Human chorionic gonadotropin (hCG) made by Organon.

A large, mature tertiary follicle that will respond to an adequate LH surge or injection of human chorionic gonadotropin (hCG) by undergoing ovulation, releasing its egg. Produces estradiol and, with exposure to LH or hCG, progesterone.

Human chorionic gonadotropin (hCG) made by Serono. Nowadays highly purified and should be suitable for subcutaneous injection.

Measurement of alpha fetoprotein in blood serum. High levels can indicate a birth defect involving the brain or spinal cord, such as anencephaly or spina bifida (confirmed if amniocentesis shows high AFP levels in the amniotic fluid). Low levels can indicate an increased risk of Down syndrome (or trisomy 21), trisomy 18 and, occasionally, Klinefelter syndrome, signaling the need for a karyotype of the fetus's tissues by chorionic villus sampling (CVS) or amniocentesis.

Measurement of human chorionic gonadotropin in serum: essentially a pregnancy test, but carried out more precisely (quantitatively) than is the case with a yes or no test (which is qualitative). Often carried out serially, to determine if a pregnancy is: thriving (hCG levels double every two or three days in normal early pregnancy; languishing (levels rise more slowly, seen with an ectopic pregnancy and with an inevitable miscarriage); or resolving naturally (levels that are falling). Very high levels are seen with hydatidiform mole. Part of the triple screen used to screen for Down syndrome in early pregnancy.

A treatment protocol for controlled stimulation of the ovaries using GnRH-agonists with injections of follicle stimulating hormone (FSH) for induction of superovulation in assisted conception programs involves starting the GnRH-agonist a day or two before the injections of FSH start. The advantage is one of cost: less FSH (and less GnRH-agonist) are used compared with the long protocol. The disadvantage is that luteinising hormone levels and progesterone levels can rise, possibly (in some cycles of treatment) spoiling optimal development of ovarian follicles. The GnRH-agonist is continued (in contrast to the ultrashort protocol) until follicles are mature and human chorionic gonadotropin is given to start the process of ovulation.

Intentional induction of multiple ovulations at once, using injections of follicle stimulating hormone and human chorionic gonadotropin, for assisted conception. Inevitably there is a risk of multiple pregnancy unless egg retrieval is performed.

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.