Fertilisation occurs and a ball of quickly multiplying cells
embeds itself in the lining of the uterus. In the UK pregnancy
is calculated from the first day of the woman's last period so
for as much as three weeks of this first month she might not be
actually pregnant. When fertilisation does occur the tiny mass
of cells called a blastocyst at this stage embeds itself in the
lining of the womb which is already thickening to support it.
The mass of cells is developing fast and becomes an embryo. For
many women the first sign of pregnancy is a missed period.
Shopbought tests are considered largely reliable so the mother-
to-be does not have to have her pregnancy confirmed by her GP.
If a first test is negative a second one a few days later may
prove positive as hormone levels in the urine rise. The embryo
officially becomes a foetus. It is about the size of a baked
bean and its spine and nervous system begin to form. The foetus
already has its own blood system and may be a different blood
group from its mother.