An unborn baby. The product of conception from the time the embryo is fully formed (from head to limbs - about 8 weeks from the last menstrual period) - until delivery. Sometimes spelt foetus.

An ectopic pregnancy located in the peritoneal cavity, or abdomen, usually having undergone implantation in the fallopian tube as a tubal pregnancy, then continuing to grow after the tube eventually ruptures. Rare in countries with ready access to medical care, as considerable symptoms have to have been ignored (and survived) during the tubal rupture part of the process. The fetus can grow almost to full term, but delivery of a live baby (delivered by operation) has never been reported.

A form of albumin (a protein in the blood) produced only by the fetus, but crosses the placenta and so is detectable in the mother's blood (measured with a serum alpha fetoprotein, as well as being usefully measured in amniotic fluid obtained by amniocentesis. Detectable in higher than usual concentration with certain open abnormalities involving the fetus's brain and spinal cord (namely anencephaly and spina bifida). Present in lower than usual concentration when there is a trisomyin the fetus, such as Down syndrome (or trisomy 21), trisomy 18 or possibly Klinefelter syndrome -- screening for all of which can be done with a triple test.

Sampling the fluid from the amniotic cavity after about 14 or 15 weeks of pregnancy. Cells from the fetus can be set up in culture for a karyotype, or for special diagnoses can be examined more quickly by fluorescent in situ hybridisation (FISH), comparative genomic hybridisation (CGH) or by polymerase chain reaction (PCR). Other substances in the amniotic fluid (such as alpha fetoprotein, or AFP) can be measured to indicate whether the fetus is normal or not.

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.

The fluid in the amniotic cavity, or gestational sac, which contains cells derived from the fetus. Sampled with amniocentesis.

Complete or partial resistance to the action of androgens in the body's tissues. Prevents or limits the development of male characteristics in the developing fetus, resulting in intersex.

Most commonly done before an operation that could cause significant loss of blood, especially if an ectopic pregnancy is suspected, because a blood transfusion might be needed. Also done for investigation of recurrent miscarriages, when the rare but important antibody anti-TjA needs to be excluded or detected. Rh-negative women who have a Rh-positive partner who are treated for miscarriage or for ectopic pregnancy often require an injection of Rhogam or anti-D gamma globulin to avoid being sensitised (developing antibodies to) possibly Rh Positive red blood cells from the embryo or fetus.

The condition when an individual shows two or more genetically different cell lines that are derived from different zygotes, because of intermingling (see microchimerism). Chimerism on a micro scale is more common than generally appreciated, in the sense that most mothers who have had a child will, on careful investigation, be shown to have some white blood cells derived from their fetus or fetuses. See also mosaicism and mixoploidy, although most cases of chimerism do not cause an abnormality of the karyotype.

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.

When the embryo or fetus is abnormal because of a mistake in its number of chromosomes (see aneuploid and polyploid), with too much or too little genetic material. Usually causes miscarriage. Revealed by performing a karyotype on the products of conception.

Abnormal development of a body part during the life of the embryo or fetus, usually but not always apparent at or soon after birth. It can be genetically inherited or be acquired by exposure to a physical or chemical insult, such as the action of a teratogen (a drug or other substance in the environment), during development in the mother's uterus. Few congenital abnormalities can be associated with a specific cause, and an apparent cause might not be the true one.

(CAH) Enlargement and abnormal function of the adrenal glands, usually from before birth, owing to a genetically determined, partial block in the production of cortisol, the adrenal glands' main hormone, resulting in overproduction of subsidiary hormones, including androgens, tending to virilise the female fetus, which can present at birth with intersex. In mild forms does not manifest until puberty, when the symptoms of oligomenorrhea and hirsutism mimic those of polycystic ovary syndrome. See also serum 17-hydroxyprogesterone.

A progestogen that is particularly effective at blocking the effect of male sex hormones (androgens)on the skin, and therefore reducing abnormal hair growth (hirsutism) and acne. Found singly in Androcur and in combination with an estrogen in Diane-35, a formulation used for oral contraception. Dangerous if taken in pregnancy, because it stops male fetuses from developing normal genital organs. Not available in the US.

The natural process in which the older the female fetus, girl or woman gets the fewer are the eggs (as primordial follicles) left in the ovaries; the huge majority of eggs are lost because of atresia, only a tiny fraction by ovulation. Before the eggs are depleted there is a mild or moderate elevation in serum FSH when measured during the menstrual phase (often called a day 3 FSH. When the eggs are more or less depleted there will be primary ovarian failure and, in women who have had periods, the menopause will take place, perhaps prematurely (premature menopause). Infertility, however, usually precedes total egg depletion by up to 10 years. See also mitochondrion and oopause.

The word is used loosely to describe everything from a fertilised egg (or zygote) to a fetus, including the embryo. What nowadays we call the embryo has for long been called the ovum by professional embryologists.

Literally, pathology of the embryo (or fetus). Can underlie a miscarriage or, if it reflects are more or less permanent state, can cause unexplained infertility.

A natural or pollutant substance in the environment possibly capable of acting -- alone or in combination with other substances -- as an estrogen or as a blocker of androgens during development of the fetus. Suspected of causing endometriosis and oligospermia in later life.

A controversial and emotionally hazardous way of dealing with a higher-order multiple pregnancy (such as quadruplets, quintuplets, or higher) in which, all the embryos or fetuses are at risk of being lost before viability. The technique involves carrying out transvaginal ultrasound and injecting a lethal substance (such as air or a solution of potassium) into the visibly beating heart of one or more of the embryos, so reducing the number of surviving embryos to three, two or one. Generally regarded as a more stressful procedure than even an induced abortion, both for the person undergoing the operation and for the ultrasound doctor asked to do it. Few people regard the availability of fetal reduction to mean that the greatest care does not need to be taken to avoid higher-order multiple pregnancies in assisted conception programs. There's a hazard: loss of the remaining fetuses from miscarriage; but the risk of this, with an experienced ultrasound doctor or fetal medicine specialist, is low.

Human leukocyte antigens, also known as tissue typing molecules, responsible for acceptance or rejection of tissue grafts, and implicated in the survival or rejection of the fetus in pregnancy.

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.

A hydatidiform mole in which there is no embryo or fetus. The size of the uterus is often larger than expected from the date of the last menstrual period, often with a high level of serum hCG; there may be bleeding; and there is a classic abnormal appearance on transvaginal ultrasound. Caused by pregnancy following abnormal fertilisation in which the oocyte is 'empty', that is it loses its pronucleus, and either (1) the male pronucleus from a single fertilising sperm doubles (the karyotype of the mole is then always 46,XX, because 46,YY is immediately lethal) or (2) there are two male pronuclei from two fertilising sperm (in which case the karyotype is either 46,XX or 46,XY). Treatment is by vacuum curettage, but there is a persisting danger of cancer (choriocarcinoma) and specialist medical follow-up is essential.

Abnormal conceptus in which there is coexistence of a hydatidiform mole with a fetus; caused by triploidy. Not as dangerous as a molar pregnancy without a fetus (see hydatidiform mole, complete), but medical follow-up is still necessary.

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 signalling molecule that inhibits the immune system, so preventing rejection of the fetus during pregnancy. It is thought that insufficient interleukin 10 is a cause of miscarriage.

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.

A duct on each side of the developing embryo or fetus that leads from the embryo's mesonephros, or temporary kidney. In male fetuses the duct persists as the Wolffian duct to form the rete testis, the epididymis, the vas deferens and the seminal vesicle. In female fetuses it usually disappears completely (in favor of the paramesonephric duct, better known as the Mullerian duct), though portions of mesonephric duct can remain as harmless cysts, called Gartner's duct cysts.

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.

The delivery of (or the process of delivering) a conceptus before there is a viable fetus.

Repeated loss of pregnancies at different stages of development of the embryo or fetus, i.e. at different stages of pregnancy. Related to recurrent miscarriages.

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 polymorphism affecting the metabolism of folic acid with a number of possible consequences avoidable by increasing dietary supplements to mega-doses (5 mg to 10 mg per day), including promotion of blood coagulation, or thrombophilia, abnormalities of the fetus (birth defects, especially neural tube defects such as spina bifida and anencephaly), recurrent miscarriage, an increased chance of heart disease and coronary thrombosis, and in men, possibly contributes to oligospermia and infertility.

White blood cells or lymphocytes that form part of the 'innate' immune system capable of recognising foreign cells and dissolving them (hence their name: natural killer cells). Found normally, however, in the endometrium of the late secretory phase and in the decidua of pregnancy, where they have more friendly purposes in forming the boundary between mother and fetus. Unlike T cells and B cells, most NK cells do not have 'immunological memory', whereby previous contact with a particular antigen causes a faster, more deadly immune response second time round.

A measurement done at transvaginal ultrasound of tissue in the neck of the fetus; if excessive, it implies a substantially increased risk that the fetus is affected by Down syndrome and Turner syndrome.

The multiplication (by the process of mitosis) of ova, or eggs, in the ovaries of the fetus.

The earliest recognisable form of the ovum, or egg; present only in the ovaries of fetuses. Multiplies by the process of mitosis before developing into oocytes by the process of meiosis. The male equivalent (the spermatogonium) normally persists in the testicles till old age. Plural, oogonia.

The female germ cell, or egg, from the earliest stage (the oogonium in the fetus), through its release from the follicle (ovulation), and (to professional embryologists) through fertilisation up to and sometimes beyond the stage of implantation. Plural: ova.

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'.

An abnormality of the embryo (or fetus) that arises after implantation of the embryo in the endometrium (the lining of the uterus). Might, sooner or later, cause a miscarriage or birth abnormality.

A syndrome occurring in late pregnancy marked by an increase in blood pressure, swelling of the ankles by fluid, and the appearance of albumin in the urine, associated with reduced blood flow to the placenta, therefore putting the fetus at risk of death, or stillbirth, and putting the mother at risk of complications from high blood pressure, convulsions (eclampsia), kidney failure, liver failure and death. Treated with drugs to lower the blood pressure and to prevent convulsions, while expediting the delivery of the baby.

An abnormality of the embryo (or fetus) that arises before the embryo undergoes implantation in the endometrium (the lining of the uterus). Usually causes miscarriage.

The form of the ovum, or egg, produced in the ovaries of fetuses by oogonia that have begun the first part of the cell division known as meiosis (by which the chromosomes will eventually halve in number). Persists into childhood and adult life by containment in follicles. Gives rise to a secondary oocyte and the first polar body just before ovulation.

Cells in the testis lining the testicular tubules that nurture the developing sperm cells; responsible in fetal life for the production of anti-Mullerian hormone, which stops the male fetus from developing fallopian tubes, a uterus and a vagina. A source of estradiol in men.

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.

A general term used by obstetricians or pediatricians to refer to a fetus or newborn baby that seems smaller than it should be for the assumed duration of the pregnancy so far. The three main causes are a genetic abnormality of the fetus; insufficient nutrition for an otherwise normal fetus; and an incorrect calculation of the dates, maybe because ovulation and conception took place later than the usual two weeks after the last menstrual period. Recent research shows that women with infertility might be at higher risk of small-for-dates babies when they do get pregnant.

A relatively undifferentiated cell in any tissue responsible for replenishing cells lost from a tissue through normal wear and tear, in the case of adult stem cells, or responsible for virtually all of a fetus's cells, in the case of embryonic stem cells; characterised by being able to replicate (through mitosis) seemingly for ever, either in the body or in tissue culture, without wearing out the ends of the chromosomes. Embryonic stem cells are controversial because they are derived from human embryos, sometimes by the process of somatic cell nuclear transfer, which involves creating embryos or embryo-like forms of a kind that could be used for reproductive cloning. The word stem comes from analogy with the stem of a plant giving rise to branches and further branches, representing the ever-increasing number of different paths a stem cell can differentiate into.

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.

A screen for congenital abnormalities of the fetus done on the mother's blood serum during pregnancy to look for fetal trisomy, including Down syndrome. The original test comprised measurements of (1) serum alpha fetoprotein (decreased in the trisomies), (2) serum hCG (increased in trisomy 21, decreased in trisomy 18), and (3) serum estriol (decreased in trisomy 21, increased in trisomy 18). Note that only 60% of pregnancies with Down syndrome will be revealed by a triple test. Refinements have taken place, with 'free beta' (part of the hCG molecule) replacing hCG itself, and with the addition of PAPP-A replacing estriol, increasing the sensitivity and specificity of the test. Further gains in sensitivity and specificity comes from adding an ultrasound test for nuchal translucency. Even so, false positive tests are possible, which will be resolved only by performing CVS or amniocentesis, and so also are false negative tests, which means that CVS and amniocentesis should still be considered in otherwise high-risk situations.

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).

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.

A sac-like structure attached to the under-surface of the developing embryo. The first part of the embryo to become visible on transvaginal ultrasound after the gestational sac, but not by itself as reassuring as seeing a beating fetal heart that the pregnancy will most likely avoid miscarriage. Not as important for a human embryo's nutrition as it is for a fertilised egg incubated by a hen, and in mammals contains no yolk, but the yolk sac's lumen is continuous with the lumen of the embryo's developing gut, or intestine, and shares an absorptive function. Cells from the yolk sac become incorporated into the fetus in many ways, including forming the primitive germ cells.