4th week of pregnancy

  • 8th to 14th day of embryonic development

You probably don’t yet know that you’re pregnant, and it is only at the end of this week that a routine pregnancy test can show a positive result. Nevertheless, a lot has happened, as the blastocyst finishes its nidation at the start of the 4th week.


There may have been menstruation-like bleeding during this implantation, which is called nidation bleeding. This can easily be confused with menstrual bleeding and cause an incorrect calculation of the length of the pregnancy. Only approximately half of all blastocysts ever develop: the others become unnoticed miscarriages due to malformations, such as chromosomal malformation, lack of hormones or circulatory problems.


The blastocyst has two different parts, the outer trophoblast and the inner embryoblast. The inner component will form the embryo and foetal membrane (with the amniotic fluid), the other component will form the placenta. The amniotic sac will function like padding and protect your baby from injuries, and will usually only open during labour.


The tiny embryo develops into three different cell layers. One cell layer forms the brain, the nerves, the sweat glands, hair and nails, the skin and the eyes and ears. Heart and circulation (blood and blood vessels), kidneys, the spleen, sexual organs, bones, muscle and deeper skin tissue will all develop from the second layer. The third layer forms the lungs, the liver, the pancreas, the thyroid gland, the urinary tract and the digestive system. And all this on not much more than one millimetre!


The placenta at this stage is called the chorion and consists of tufts of membrane with tiny tree-like extensions, the chorionic villi, which envelop the embryo. The chorion is connected to the embryo through a stem that later becomes the umbilical cord. The chorionic villi connect to the mother’s blood vessels and thus make the most important function of the placenta possible: the exchange of oxygen, nutrients and waste products between the embryonic and maternal circulations. The placenta also produces many different hormones - for example HCG (human chorionic gonadotropin, which is also called the ‘pregnancy hormone’), oestrogen and progesterone – which make sure no menstruation is triggered and that pregnancy is continued.


The placenta is also important for the growth of the uterus and for changes in the breasts in preparation for breast-feeding. Additionally, it serves as a barrier against many (but unfortunately not all) harmful influences, such as certain infections.


A high progesterone level exerts a strong relaxing influence on the uterus muscles in all these processes in order for the embryo not to be pushed out. Unfortunately, the rest of the organism can be influenced by these hormonal changes, which reveals itself in symptoms such as fatigue, frequent urinary urges and constipation.


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