Baby: Week 1
The embryo at week 1 - from the moment of conception to 7 days
All systems Go!
Your first nine months begin today, even though you don’t quite exist yet! How can that be?
Well, it’s impossible to narrow down the exact day or hour of conception, so doctors date the pregnancy from a starting point they can actually calculate—the first day of the mum’s last menstrual period. There’s approximately a two-week time lag, but again the dates are not exact, so if mum is four weeks pregnant, her baby is about two weeks old.
What happens right before you are conceived? Your mother’s body makes your new home as welcoming and nourishing as possible! This week a hormone called oestrogen creates a blood-rich lining in the womb. Progesterone also rises to prepare the uterus for you. 
Also, the body prepares for pregnancy to actually occur. This week one of mum’s eggs will leave the ovary and enter the fallopian tube. If one of dad’s 350 million sperm swims up to meet the egg, it will be fertilised and new life will begin!
Day 1: Conception
Conception takes place and life begins. The sperm has raced upstream to meet the egg and fertilise it, forming a fertilised egg or zygote. From the moment of conception (also known as fertilisation), 46 chromosomes with 30,000 genes combine to determine all your physical characteristics: sex; facial features; body type; colour of hair, eyes and skin. Even more amazingly, intelligence and personality - the way you think and feel - were already in place within your genetic code. At the moment of conception you were essentially and uniquely you! The zygote’s sex is already determined even at this stage by its chromosomes. One X chromosome always comes from the mother; the father provides either an X or Y chromosome. If the combination is XX, the baby is a girl; if it is XY, it’s a boy!
Filled with the genetic information from the mother and the father, you travel down the Fallopian tubes to the uterus (womb), and during that journey you grow and divide to become a blastocyst. As a blastocyst, made up of about 100 cells, you’re made up of two layers, and the outer layer will become the placenta.
Day 5: Home sweet home!
You reach the womb today. Home sweet home!
Day 6: Implantation
You implant into the thickened uterine wall.
Babycentre.co.uk discuss week 3 of foetal developmentfind out more
What to expect when you are expecting - Week 3find out more
Langmans Medical Embryology, Ninth Edition, TW Sadler, 2003find out more
WebMD discuss your pregnancy week by week - Weeks 1 - 4find out more