Fourth Week of Pregnancy: What to Expect

The fourth week of pregnancy corresponds to the fourth and last week of the menstrual cycle before the menstrual period is due. Therefore, in case of a positive pregnancy, there will be a delay in menstruation that will make the mother suspect a possible pregnancy.

Although it is very early in the pregnancy, the embryo is already more than a week old and has undergone changes in its size and structure on the way from the fallopian tubes to the uterus.

In week 4 of pregnancy will be when this embryo implants in the endometrium (inner uterine layer) and the mother begins to undergo changes in her body, such as the well-known implantation bleeding.

Because the weeks of pregnancy are counted from the date of the last menstrual period (LMP), they do not coincide with the gestational age of the fetus. In the fourth week of pregnancy, the embryo is actually only 2 weeks old since conception.

Despite this, all reproductive professionals use the LMP as a reference for counting the weeks of pregnancy in the gestation of 40 weeks in total.

Embryo implantation

In the fourth week of pregnancy, when the embryo arrives in the maternal uterus, it is in the blastocyst stage and has 2 differentiated structures: the inner cell mass (ICM), from which the baby will develop, and the trophoblast, which will give rise to the placenta.

After a couple of days or so in the uterine cavity, the embryo must implant in the endometrium for an evolving pregnancy to take place.

Embryo implantation is a complex process that consists of the adhesion of the embryo to the endometrium in order to receive the blood with oxygen and nutrients necessary for its subsequent development by the mother.

Formation of the placenta

The placenta is the organ that connects the baby to the mother and is responsible for its nutrition during the nine months of pregnancy.

Its formation begins with embryo implantation and culminates in the fourth month of pregnancy when it finally acquires its full functionality.

For this purpose, the trophoblast of the blastocyst, also called trophoectoderm, is differentiated into the following layers:

• Syncytiotrophoblast (outer layer): its cells erode the maternal capillaries of the endometrium, blood flows and a utero-placental circulation is established.
• Cytotrophoblast (inner layer): its cells proliferate in the syncytiotrophoblast forming the primary chorionic villi. The cells migrate, proliferate, and form the definitive yolk sac.

With all this, blood vessels develop, and the blood volume increases by about 50% to cope with the oxygen demand of the fetus. In addition, the attachment fragment of the embryo to the endometrium will become the umbilical cord.

The fetus and the amniotic cavity

Before implantation, the blastocyst detaches from the zona pellucida that surrounds it in a process called hatching, which will allow it to establish connections with the endometrium in order to attach to and invade it.

Once implantation has taken place, the embryo measures approximately 1 mm and the amniotic cavity begins to form, a kind of sac limited by a membrane called the amnion, inside which the fetus will develop, suspended in the amniotic fluid.

The functions of the amniotic fluid with respect to the fetus during pregnancy are as follows:

• To protect the fetus
• To maintain it at a suitable temperature
• To allow it to grow symmetrically
• To allow it to move freely
• To promote the development of the lungs

In addition to the amniotic cavity, the yolk sac or yolk vesicle, an embryonic attachment that serves the function of nourishing the fetus before the complete formation of the placenta, is also formed.

In week 5 it is already possible to see the yolk sac within the gestational sac by ultrasound, even before the embryo, and it usually disappears by the end of the first trimester.

When a woman discovers she is pregnant, she should change her habits and adapt her daily routine to this new state.

Among the multitude of advice and guidelines that specialists can give for a healthy pregnancy, we highlight the following in terms of food and sport:

• Taking folic acid helps prevent the appearance of congenital defects in the brain and spinal cord in the baby. A daily supplement of 400 mg per day is usually recommended.
• Increase consumption of dairy products skimmed or semi-skimmed milk and yogurt, as well as semi-cured cheese, provide the calcium necessary for the development of the baby’s nervous and muscular system.
• Adapt physical exercise to reduce the intensity of physical activity or, if you do not practice any sport, begin to do gentle physical exercise to improve muscle tone, strength, and endurance, which will be beneficial throughout pregnancy and during childbirth.
• Foods to eat vegetables, fruits, and vegetables that provide fiber. Also, meat and fish provide the necessary amount of iron and omega-3 acids.
• Substances to avoid coffee, tobacco, alcohol, or other drugs.

As soon as a woman knows she is pregnant, the first thing she should do is call her gynecologist to make an appointment and have the first ultrasound.