During this 3rd week of pregnancy (3 SG), ie the 5th week of amenorrhea (5 WA), the development of the egg accelerates. Over the course of successive cell divisions, the egg grows and is now 1,5 mm. It has an ovoid shape: the wide end corresponds to the cephalic region, the narrow one to the caudal region (lower part of the body).

Then begins an essential process, during this 1st month of pregnancy: cell differentiation. It is from each cell of this period that all the other cells of the baby will be derived. From the 17th day, the embryonic disc begins to thicken at its midline, along the head-tail axis. This is the primitive streak that will elongate and occupy about half the length of the embryo. From this primitive streak a new layer of cells will differentiate. It is gastrulation: from didermic (two layers of cells), the embryonic disc becomes tridermal. It is now made up of three layers of cells, the source of all the baby’s organs:

The inner layer will give the organs of the digestive system (intestine, stomach, bladder, liver, pancreas) and the respiratory system (lungs);

· From the middle layer are formed the skeleton (except the skull), muscles, sex glands (testes or ovaries), heart, vessels and the entire circulatory system;

· The outer layer is at the origin of the nervous system, the organs of the senses, the skin, the nails, hairs and hair.

Some organs come from two layers. This is particularly the case with the brain. On the 19th day, one of the ends of the primitive streak presents a swollen part towards which different cells have migrated: it is the outline of the brain, from which the whole central nervous system will be built during the process called neurulation. On the back of the embryo, a kind of gutter is hollowed out then forms a tube around which protuberances appear, the somites. This is the outline of the spine.

The placenta continues to develop from the trophoblast, whose cells multiply and branch to form villi. Between these villi, the gaps filled with maternal blood continue to merge with each other.

Last but not least, a big change: at the end of the third week of pregnancy the embryo has a heart which beats, admittedly gently (about 40 beats / minute), but which beats. This heart, which is still only a cardiac outline formed of two tubes, was formed from the primitive streak between the 19th and 21st days, when the embryo is almost 3 weeks old.

It is during the 5th week of amenorrhea (3 SG), that the first sign of pregnancy finally appears: the delay of rules.

At the same time, other signs can appear under the effect of the hormonal climate of pregnancy, and more specifically of the hormone hCG and progesterone:

• a swollen and tense chest;
• tiredness ;
• frequent urge to urinate;
• morning sickness;
• some irritability.

Pregnancy is however still invisible during the 1st trimester.

Even though symptoms can be subtly felt when a woman is 3 weeks pregnant, new lifestyle habits need to be adopted. This allows the fetus to develop in good conditions. The mother-to-be must take her needs into consideration, especially taking care of herself and avoiding stress. Fatigue and anxiety can indeed be harmful to the 3 week old embryo. To remedy this, the pregnant woman can take naps if she is sleepy during the day. Also, relaxation exercises, such as meditation or a calming activity, can help you feel good and serene. It is also recommended to practice gentle physical activity, such as walking or swimming. A medical opinion can be requested from his doctor. 

The in-vitro baby will be able to feed through the placenta. Food is therefore very important throughout pregnancy, with foods to be favored according to the different stages. At 5 weeks of amenorrhea (3 SG), folic acid is essential for the good development of the baby. It is vitamin B9, essential for cell multiplication. Folic acid is also actively involved in healthy brain development. Indeed, at 3 weeks of pregnancy (5 weeks), the formation of the embryo’s brain has already started. 

Vitamin B9 is not made by the body. It is therefore necessary to bring it to him, even before conception and then throughout the first month of pregnancy, and even beyond the second month of pregnancy. The goal is to avoid a deficiency that can weaken the growth of the fetus. This can be done with a supplement or with food. Some foods are high in folic acid. This is the case with green vegetables (spinach, cabbage, beans, etc.). Legumes (lentils, peas, beans, etc.) also contain it. Finally, certain fruits, such as melon or orange, can prevent possible folic acid deficiencies. 

When you are pregnant, it is important to eat balanced meals and not indulge in sweets or processed foods. These have no nutritional interests and facilitate weight gain in the expectant mother. It is recommended to drink between 1,5 L and 2 L of water daily because the blood volume of the pregnant woman increases. In addition, hydrating well helps provide minerals and prevent urinary tract infections or constipation.

From the first day of late period, it is possible to take a pregnancy test, preferably on the morning urine which is more concentrated. The test is reliable at 3 weeks of pregnancy (5 weeks). 

A blood test will then be necessary to confirm the pregnancy. It is advisable to make an appointment quickly with your gynecologist or a midwife in order to plan the first compulsory prenatal visit. This first official visit can be made until the end of the 3rd month of pregnancy (15 weeks), but it is preferable to do it early enough. The first prenatal examination indeed includes different serologies (toxoplasmosis in particular) of which it is important to know the results in order, if necessary, to take the necessary precautions on a daily basis.

The first weeks of pregnancy take place organogenesis, a stage during which all the baby’s organs are put in place. It is therefore a high-risk period, as exposure to certain substances can interfere with this process. As soon as the pregnancy is confirmed, it is therefore necessary to stop all risky practices: smoking, consumption of alcohol, drugs, taking medication without medical advice, exposure to X-rays. Different aids exist, in particular for smoking cessation. Do not hesitate to talk to your gynecologist, a midwife or your doctor.

Bleeding is frequent at the beginning, during the 1st month of pregnancy, but fortunately does not always signal a miscarriage. However, it is advisable to consult in order to check the good progress of the pregnancy. Likewise, any pelvic pain, especially sharp, should be consulted in order to rule out a possible ectopic pregnancy.