2nd week of pregnancy

During the 2nd week of pregnancy, the embryo begins implantation, that is to say, to settle well on the wall of the uterus of the future mother. Once properly installed, it can grow.

The 2nd week of pregnancy is 4 weeks of amenorrhea (no menstruation). Indeed, in France, gynecologists usually count the duration of pregnancy in weeks of amenorrhea (SA). Thus, the calculation of the pregnancy begins on the 1st day of the last period, so about 2 weeks before the fertilization (before the actual beginning of the pregnancy). Because it is only in the middle of the woman's cycle, around the date of ovulation, that the egg can be fertilized by a spermatozoon. And so in reality, the 1st week of pregnancy is the 3rd week of amenorrhea (SA).

At the beginning of the 2nd week of pregnancy (4 weeks) approximately the implantation of the embryo occurs.

Before implantation, the fertilized egg (future embryo) floats for about 3 days in the uterus. When the fertilized egg fails to settle, it will be evacuated during the next period. When nesting, on the other hand, we speak of "successful" design.

How is nesting going?

Under the influence of hormones (estrogen then progesterone), the lining of the uterus will thicken. On the 7th day of life, the embryo adheres and gradually makes its nest in the thickness of the mucosa. Where the embryo attaches, vessels multiply and glands secrete nutrients. Small filaments, the chorionic villi, sink into the lining of the uterus. This is the future placenta that how to secrete pregnancy hormones whose rate doubles every 24 hours: these hormones, called ßHCG (hormone chorionic gonadotropin), are sought and measured by performing a pregnancy test.

However, you should know that at the 2nd week of pregnancy (4 weeks), a pregnancy test can not necessarily give a positive and relevant result. It is only at the end of the 4th week of amenorrhea, that is to say during the 5th SA, that a urine test provides a generally reliable result. In case of negativity, and if you want to be certain of the diagnosis of pregnancy or not, it is advisable to repeat this test 6 days later, or to have a blood test or urine test in a laboratory.

Mom side

At the 2nd week of pregnancy (4 weeks), the uterus prepared for the arrival of the fertilized egg. The uterine lining has thickened to provide optimal protection: the egg can implant.

The hormones that are produced by the placenta and that direct the great physical and physiological changes in the woman during the first weeks of pregnancy, can already cause some typical signs of pregnancy. Indeed, even without knowing that they are pregnant, some women perceive from the 2nd week of pregnancy (4 weeks) pregnancy symptoms. These symptoms, very diverse, can go from the stuffy nose to more salivation, a great tiredness to nausea .. not to mention the famous emotional roller coaster (which is not always easy for the entourage).

But typically, typical pregnancy signs occur later, around the 6th week of pregnancy (8 weeks). Most often, these symptoms occur during the first trimester of pregnancy, but decrease sharply during the 2nd trimester of pregnancy, that is to say around the 11th week of pregnancy (13 weeks).

Hormones also cause enlargement of the breast and uterus. Especially breast swelling is a phenomenon that pregnant women often feel very early: the breasts can become hard and "tug".

During the 2nd week of pregnancy (4 weeks), slight bleeding (bleeding nidation) may occur. Many women confuse bleeding with mild menstrual bleeding. These bleeds come from lesions of small vessels in the uterus, caused by the egg during its implantation.


If you smoke, an immediate and total stop of the cigarette is strongly advised. Because at this stage, the embryo is particularly exposed to different risks. The same goes for alcohol and drugs that are particularly harmful and dangerous for the embryo during these first weeks of pregnancy.

It is important to consult with any suspicious symptoms or pain that you observe during your pregnancy. Although most symptoms may be harmless, it is best to contact the midwife or doctor.

Also, avoid taking medications or other products without talking to your doctor, even if they are natural or homeopathic products. Some herbal infusions, for example, can cause premature contractions.

Baby side

On average, implantation takes place on the seventh day after fertilization: the fertilized egg that is now called a blastocyst nests in the uterus. During this phase, the embryo is particularly fragile, external influences can strongly affect it. If the embryo succeeds in repairing this damage, the pregnancy continues. If not, the process of development - and therefore pregnancy - stops.

But for the embryo to nest in the uterus, it enters the uterine lining until it completely surrounds it: a prerequisite for the exchange of biochemicals, nutrients and of oxygen between the mother and the embryo.

Between the cells that will form the embryo and placenta, appear a small amniotic cavity and the vesicle vitelline which is, initially, responsible for the nutritional supply. The placenta that will support this function later, gradually forms around the egg.

Until the 2nd week of pregnancy (4 weeks), the cells of the embryo are pluripotent, which means that any organ can form from each cell. It is only from the 2nd week of pregnancy that cells begin to specialize.

The cells that make up the embryo (embryoblasts) will be organized into an embryonic disk that will initially consist of two, then a little later of three embryonic leaves:

- internal embryonic leaflet: endoderm,
- intermediate embryonic leaflet: mesoderm,
- external embryonic leaflet: ectoderm.

Nesting is now complete. The embryonic disk, the amniotic cavity and the yolk sac are completely covered by the uterine lining: only by targeted research can a tiny bulge be discerned.

At the 2nd week of pregnancy, the embryo measures just 0.5 mm, it begins to grow. In late pregnancy, the baby's weight averages 3, 300 kg and a height of about 52 cm.

The placenta

The placenta is a vital organ for the baby: it is he who allows all the exchange of substances between mother and fetus. The placenta is the nutritional organ of the fetus, which transmits vitamins, nutrients, trace elements and oxygen from the mother's blood to the fetus. In the opposite direction, the placenta also allows the evacuation of carbon dioxide and the metabolic waste of the fetus.

This organ, which exists only during the pregnancy, develops in parallel, as the baby grows up: the placenta is perfectly tuned to the needs of the fetus. The placenta tissue, which looks like a sort of sponge, houses the blood vessels that connect the fetal and maternal circulation, located on either side of the placenta.

In addition to feeding the baby, the placenta also has a protective function by preventing most bacteria and harmful substances from passing. On the other hand, it can not filter alcohol and drugs and drugs that reach fetal blood so it is important to avoid consumption. Or for the drugs to do it by following the medical advice.

Shortly after delivery, the placenta is expelled (this is deliverance).

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Authors: Dorothee Gebele, Dr. Nicolas Evrard

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