(LMP) The normal period immediately preceding conception. For convenience, obstetricians calculate the duration of a pregnancy as starting with this date, even though of course it does not truly begin until fertilisation (or, some would say, implantation).

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.

A fluid-filled bag of membranes in which the embryo forms during pregnancy. Visible on transvaginal ultrasound from about 5 weeks from the last menstrual period. Technically, the amniotic cavity (and later in pregnancy able to be sampled with amniocentesis).

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.

A small to moderate amount of vaginal bleeding at the time that implantation becomes established. Implantation bleeding can be confused with a menstrual period (particularly the last menstrual period) to give a false impression that pregnancy has not happened or, later, a false estimate of the duration of the pregnancy.

The onset of labor, leading to delivery, before at least 37 weeks have elapsed since the last menstrual period. The more prematurely babies are born, the greater the difficulty they have surviving, even with expert care.

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.