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CONCEPTION TO BIRTH
The magical journey from conception to birth takes an average of 40 weeks and some remarkable changes happen as the fertilised egg divides, the cells of the embryo specialise and the foetus takes the recognisable shape of a new baby.
Follow this journey to see how we take form.
EGG MEETS SPERM
Week 1
It takes just one of the many millions of sperm swimming toward the egg as it comes down the fallopian tube to achieve fertilisation. Fertilisation is when sperm enters the egg and genetic material mingles – it’s the moment of conception where the nine-month journey towards the birth of a new baby begins. It is at this point that the sex of the baby is determined, depending on whether the father’s sperm carries an X chromosome in which case the baby will be a girl, or a Y chromosome and the baby will be a boy.
IMPLANTATION
Week 2
Cell division begins within a few hours of fertilisation. By three days the zygote (fused egg and sperm) has developed into a solid ball of around 16 cells, which travels down the fallopian tube towards the uterus.
Meanwhile, hormones produced by the ovaries (oestrogen and progesterone) cause the lining of the uterus (endometrium) to thicken and prevent any more eggs from being released (ovulation).
Cell division continues and by six to seven days the zygote develops into a hollow structure of around 100 cells called the ‘blastocyst’, which contains a mass of very special cells at its core, from which the embryo will develop.
Entering the uterus, the blastocyst “hatches” by breaking free of its restrictive outer membrane, and begins to implant by burrowing into the uterus lining. As it implants deeper into maternal tissue, the new life forms small finger-like projections (or villi) which tap into the mother’s blood supply, eventually forming the placenta and umbilical cord, which will provide for the needs of the developing foetus.
A hormone called 'human chorionic gonadotrophin' or HCG (the 'pregnancy hormone') is produced by the implanting cells, to maintain the blood-rich endometrium and prevent it from being shed, as happens during a normal menstrual cycle.
If the blastocyst does not reach the uterus in time, it may implant into the fallopian tube wall and this is called an ectopic pregnancy.
EMBRYONIC DEVELOPMENT
Weeks 3-8
The cells of the pre-implantation blastocyst are also known as embryonic stem cells because at this stage all cells are the same. Once cells begin to differentiate or specialise, they develop into specific cell types (such as skin cells, kidney cells, heart cells, etc) ready for their function and role in life.
After implantation. the blastocyst is officially an embryo and becomes enclosed in a fluid-filled amniotic sac, which protects the developing embryo and acts as a shock absorber. The outer membranes of this sac form part of the placenta.
Massive changes happen during the next eight weeks, with the development of specialised cells and tissues:
- Three distinct layers of germ cells develop: an outer layer forming skin, hair and nervous system; a middle layer forming muscle, bone, blood vessels and essential organs like the liver; and an inner layer forming digestive, excretory and respiratory systems.
- The yolk sac, which grows from the embryonic cells of the blastocyst, forms part of the primitive digestive tract and is also where foetal blood cells are first made.
- A stalk develops into the umbilical cord.
- A tube of primitive tube nerve cells folds inwards as it develops into the spinal cord and brain.
- Limb buds begin to sprout and then grow into arms and legs.
- The heart develops and begins to pump.
- Facial features appear.
GROWTH OF THE FOETUS
Weeks 9-23
By now most of the developmental stage is over and the growth phase begins. The foetus is two to three centimetres long and grows by six times its size over the next two months.
The placenta, made from a mixture of maternal and foetal cells, grows and provides the foetus with a source of nutrients and oxygen from the maternal blood, and a waste disposal service via the umbilical cord, although foetal and maternal blood never mixes. The placenta allows protective maternal antibodies to reach the foetus and also acts as a filter for harmful substances, although some still get through, such as alcohol, nicotine and other drugs. The hormones progesterone and HCG continue to be produced by the placenta to maintain the pregnancy.
During the next few months the foetus grows rapidly and so does the uterus, swelling as a visible sign of pregnancy. The foetal heart can be heard through a stethoscope by 20 weeks. A first ultrasound scan is usually taken a little earlier and gives a clear image of the baby, complete with heart, organs, and sex of the baby, if you know where to look or want to know!
At around 20 weeks fingernails, eyelashes and hair begin to grow; the senses become fully developed and the foetus can detect sound and light and then begins to move.
PREPARATION FOR BIRTH
Weeks 24 -40
The final stages of physical development take place; the bones harden apart from the skull which stays soft until after birth. The body becomes covered in a fatty substance and fine downy hair which protects the skin. At 24 weeks the lungs are fully formed and ready to breathe, although they are not needed until the moment of birth. At this stage, mother can feel the baby moving.
As the final stages of pregnancy approach, the baby fills the uterus and is active, although still coiled in the typical foetal position. Oestrogen (still in full production by the ovaries under the influence of placental progesterone) causes softening of the pubic joints and ligaments in preparation for labour. The maternal blood volume increases to compensate for blood loss during childbirth.
Two to three weeks before labour begins, the cervix softens, the lower part of the uterus expands and the baby begins to move down into the pelvis and into position at the entrance to the cervix or birth canal.
BIRTH
Eventually everything is in place and the fully developed foetus is now a baby ready to leave the safe environment of the womb and enter its new world outside. Falling levels of the hormone progesterone and increasing pressure of the baby at the head of the cervix are thought to trigger the start of labour. The fluid-filled amniotic sac surrounding the baby breaks, shedding its “waters”, and uterine contractions herald the start of the birth process as labour begins.
