You are now 3 weeks pregnant, counted from the beginning of your last period, even though only 7 days have passed since your baby was conceived. This is the week before the woman's next period would have been due (or the beginning of week 4). The blastocyst fully embeds or implants into the lining of the woman's womb (called the endometrium). The whole implantation process takes about 6 days to complete (finishing about 12 days after fertilisation, or about 2 days before the woman's next period would have been due). Once the blastocyst is fully implanted, the baby is able to 'tap into' their mother's blood supply.
As baby and mother unite, the baby produces hormones that prevent the woman's next period from coming, the same hormones that cause a pregnancy test to turn positive (called 'human gonadotrophin hormone' or 'HCG'). In turn, the mother's blood stream starts to provide the baby with nourishment to continue to grow. Up until this time your baby has been nourished by glucose secreted from the lining of the fallopian tubes and uterus.
While implantation cannot usually be felt, some women experience a small amount of bleeding as their baby burrows into the thick lining of the uterus. An implantation bleed can be noticed around the time the woman's next period would have been due, often confusing this bleeding with having a menstrual period.
The changing blastocyst is like a ball, about 4mm in width. Inside the ball, your baby's cells clump together on one side, continuing to multiply and differentiate, then making a linear formation. This line up of cells creates a flat disc with 3 distinct layers, called the:
Endoderm. This becomes your baby's internal organs (lungs, liver, bowel and bladder).
Mesoderm. This becomes your baby's skull and bones, sex organs, muscles and heart.
Ectoderm. This becomes your baby's skin, hair, eyes, ears, brain and spinal cord.
At the same time, the outside of the blastocyst divides onto 2 layers to start forming the life support systems for your baby. The outer layer (or the 'chorion') grows fronds (or 'villi') that penetrate the rich endometrium, to obtain nourishment from their mother's blood stream (the beginnings of theplacenta). The inner layer (or 'amnion') becomes the amniotic sac, which slowly fills with amniotic fluid. Amniotic fluid surrounds your baby, allowing them to move freely as well as controlling the temperature of their environment and acting like a shock absorber to protect them.
For many women, this amazing process is all happening undetected, because they do not realise they are pregnant yet (but a few might suspect it!)
Image 66-31 shows a microscopic image of a blastocyst.
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