Month 1: In the very first few days human embryos hatch out of the fertilised egg, and a new theory is that not hatching properly probably leads to the creation of identical twins, triplets and quadruplets.
At six days old, you are just a clump of cells already transferred to your mother’s womb, needing to implant yourself into its lining. Only healthy embryos are worth nourishing for nine months; they release a chemical signal showing they are developing properly. Around two-thirds of embryos fail at this stage or soon after and are lost.
At four weeks the shape of body and limbs emerges. At eight weeks an
embryo becomes a foetus. A foetus’ age is the duration of its gestation – two
weeks more than time since conception.
Month 2: Fingers and toes start to develop under the direction of the 'Sonic Hedgehog' gene. If there are mutations in this gene, you can end up with more than 10 fingers or toes.
Over the next few weeks, 14 different structures come together to make a scaffold for intricate layers of tissue to form the face. No two faces are exactly the same; your face is the result of your genes, and the precise timing of when they were switched on and off during this critical process. Failure to align correctly produces conditions such as hare lip and cleft palate.
At 12 weeks the
layers of skin around the fingers begins to wrinkle, pushing against
the amniotic fluid surrounding them. This interaction helps mould a unique combination of arches, loops and
whorls in the fingertips. Even identical twins experience slightly
different pressures from the amniotic fluid and develop subtly different
patterns. By 17 weeks, your 10 fingerprints were complete.
Month 4: Skin is initially transparent, but now develops a fine fur-like hair called "laguno", sweat glands develop and melanocytes (that give the skin its colour) colonise the skin from the tissue beneath. An albino is someone who has no melanin; they are prone to skin cancer.
By 14 weeks you were making human leukocyte antigen (HLA) proteins, which help your immune system recognise bacteria and viruses. There are thousands of possible combinations of HLAs – you inherited your set from your parents. One theory suggests that HLA proteins change our aroma to other adults, and that we choose a sexual partner with a very different HLA makeup, and therefore smell, to our own.
Month 5: From 15 weeks, male foetuses receive a big surge in testosterone, created in their testicles. Female foetuses receive a much lower dose from their mother and their adrenal gland. Around this time, aspects of personality are being connected in the brain. Exposure to high levels of testosterone is thought to contribute to more ‘male-type’ behaviours like risk-taking. People exposed to higher testosterone in the womb also have a longer ring-finger relative to their index finger.
It is thought that the surges of testosterone affect the brain with feelings of being male or female (gender). Humans can therefore have disparate 'settings' for (a) brain sex, (b) body sex and (c) attraction. The transgender ratio is estimated to be 1 in 10,000.
If the 'wrong' testosterone (dihydro-) is produced, males do not develop a penis. They are usually brought up as girls, but puberty surges of ordinary testosterone stimulates the growth of the penis. The condition is unusual but can be common in some populations.
Month 6: Bones are initially formed as cartilage that slowly hardens to bone as pregnancy progresses. Bone cells create hard bone, laying it down like cement. At this stage most of the bones have calcified but all still have soft parts that allow you to keep growing.
The fat used by muscles starts to be laid down at 23 weeks. Carnitine transports fatty acids so they can be broken down to generate energy for the muscles; supplied in the womb by the mother, after birth is produced by your kidneys and liver and derived from meat and dairy products. Carnitine transporter disease affects parts of the process; it is 100 times more common in the isolated population of the Faroe Islands than the 1 in 40,000 times in populations elsewhere.
Month 7: Most of the brain's growth is now about wiring; fatty sheaths are wrapped around the brain cells, insulating them so they can send signals around your head. Around 100 billion new connections are made every day and the brain starts to lay the foundation for memory. Hearing is the first of the senses to develop.
By 28 weeks brain and body are well developed. Two
eyes lined with colour-sensing cone cells have developed. Pigments that
could detect short (blue), medium (green) or long (red) wavelengths of
light arre being produced. Most people can detect 10 million different
colours once born. But 8% of males and 0.5% of females are born
colour-blind, without all the necessary pigments. Some people are born
with a fourth type of pigment that senses wavelengths between red and
green, so they see colours even more vividly.
Month 8: The foetus gains weight rapidly, building up a big fat reserve. Size at birth depends on many things, including race, gender and genes. But
external factors like mother’s diet, stress levels and smoking
status also play a role. Evidence suggests that the environment in
your mother’s womb might have changed chemical markers within your DNA
that control how your genes were switched on and off as you grew, and that this might impact aspects of your health
later in life, such as body mass index, risk of diabetes and cognitive
performance. It seems that when a mother's diet is low in calories (fat & sugar), the foetus becomes very efficient in extracting nourishment, setting the body to do the same life long.
Michael Mosley in Radio Times 12-18 Sep. 2015. This is based on Mosley's Countdown to Life: the Extraordinary Making of You programme broadcast on BBC2 14, 21 and 28 Sep. 2015.
and
BBC website: Nine things that shape your identity before birth
