3 weeks pregnant – the start of the development of the embryo

By (gynecologist), (embryologist), (embryologist) and (biochemist).
Last Update: 09/07/2022

The third week of gestation coincides with the third week after the last menstrual period (LMP). Usually, the woman does not yet know she is pregnant and pregnancy tests can still result in false negatives.

Actually, this third week of pregnancy would be the first of embryonic development. However, all the professionals in the sector take as a reference the beginning of the menstrual cycle to start counting the weeks of pregnancy in a 40-week gestation. We call this gestational age.

Therefore, during this third week of pregnancy, the events that take place are fertilization and the beginning of the embryonic development.

What happens at three weeks of pregnancy?

At the end of the second week or beginning of the third week of pregnancy, ovulation takes place. These are a woman's fertile days and, therefore, the couple must have unprotected sex for conception to occur.

After intercourse, ejaculated sperm begin their journey through the female reproductive system: they ascend through the cervix, pass through the uterus and reach the fallopian tubes. This is where the meeting with the egg and fertilization by a single sperm takes place.

Beginning of the embryonic development

Once the egg and sperm have fused, their nuclei merge, where the genetic material of both parents is contained. At 16-20 hours after fertilization, the embryo consists of a single cell and is called a zygote.

From here, the zygote begins its descent down the fallopian tube towards the uterus. At the same time, the number of cells divides and increases as the days of this third week pass, as shown below:

Day 0
fertilization of the embryo by the union of egg and sperm.
Day 1
at 16-20 hours after fertilization, the zygote appears.
Day 2
the segmentation of the zygote results in a 4-cell embryo.
Day 3
the embryo has divided into approximately 8 cells.
Day 4
the embryo already has at least 16 cells and is called a morula.
Day 5
the embryo increases in size and becomes a blastocyst of about 200 cells.
Day 6
the blastocyst expands and the number of cells continues to increase.

At the end of the third week of pregnancy, the embryo reaches the uterus when it is in the blastocyst stage.

You can get more information about the nomenclature that the embryo has in its different stages in the next post: What’s the Difference Between Zygote, Embryo & Fetus?

Journey of the embryo through the Fallopian tube

As mentioned before, after fertilization embryonic development begins and the embryo descends through the Fallopian tube to the uterus.

This path is made possible by small contractions of the tubal muscles. In addition, its inner surface is covered with millions of cilia, small hairs that move in the same direction, which produces a dragging of the embryo through this conduit.

It is necessary to point out that it is at this moment when there is a risk of ectopic pregnancy, that is, the embryo embeds in the fallopian tube and an extrauterine pregnancy occurs, which is not viable.

The hormone progesterone secreted by the ovary after ovulation also plays an important role during this pathway. Progesterone, also known as the pregnancy hormone, allows the sphincter between the fallopian tube and the uterus to relax, allowing the embryo to pass through.

In addition, progesterone also prepares the uterus for embryo implantation by increasing endometrial receptivity.

Changes in the mother

In the third week of pregnancy, when conception has already taken place, the woman does not have any major changes in her body and so won't feel anything yet. In fact, in most cases she does not even suspect that she is pregnant.

Only patients who have had fertility treatments such as artificial insemination (AI) or in vitro fertilization (IVF) notice symptoms in this time, known as the Two Week Wait. These symptoms after fertility treatments are usually due to the hormonal medication taken, rather than pregnancy itself.

In spite of this, the hormonal alterations during this third week of gestation can give rise to symptoms such as the following:

  • Sensitivity in the breasts
  • Greater perception of smells
  • Alterations in taste and increased salivation
  • Mood swings and irritability
  • Fatigue
  • Headache
  • Increased urge to urinate
  • White and dense vaginal flow

Usually, this third week of pregnancy goes by without symptoms noticeable by the woman. In fact, there would still be a week left for menstruation, and women do not usually take a pregnancy test before the delay in menstruation.

Is it possible to take a pregnancy test?

If you take a pregnancy test during the third week of pregnancy, you will most likely be negative because the beta-hCG hormone has not yet begun to be secreted.

Assisted reproductive patients also run the risk of getting a false positive pregnancy test this week, as hormonal medication given during treatment can lead to a wrong result.

The beta-hCG hormone corresponds to the beta fraction of the human chorionic gonadotropin hormone, which is produced by the embryo once implantation has taken place and is detected by a pregnancy test.

In the next post, we explain when is the ideal time to take a pregnancy test: When to Do a Pregnancy Test? – How to Use, Results & Accuracy

Special care in the third week

A woman's habits and behavior can influence her baby's development from the beginning of gestation. Therefore, you should follow a healthy lifestyle, especially if it is a looked for pregnancy.

Here are some interesting tips during the first weeks of pregnancy:


Folic acid is one of the most important vitamin components for pregnant women. It helps prevent defects in the fetal neural tube, which could lead to diseases such as spina bifida and anencephaly. Folic acid is found in leafy green vegetables, such as spinach or chard, and in legumes.

During the first few weeks of pregnancy, gynecologists may recommend starting iron-rich foods or vitamin supplements. This mineral is necessary for the formation of hemoglobin and to prevent anemia. Moderate red meat is an excellent source of iron. As for foods that help the absorption of iron we find tangerines, strawberries or tomatoes rich in vitamin C.

Another very important mineral in pregnant women is calcium, because it helps to prevent hypertension or pre-eclampsia. Calcium also contributes to the optimal development of the baby, and the baby's nervous and muscular system. Therefore, a diet rich in calcium is recommended from the beginning of pregnancy.

When a woman knows she is expecting a baby, she should avoid the following foods: raw or mercury-rich fish, raw meat, coffee, undercooked eggs, unpasteurized cheese, etc.

These are all preventive measures to prevent toxoplasmosis and listeriosis. It is also necessary to wash fruits and vegetables properly before eating them.


Besides taking care of the diet, it is also important for pregnant women to maintain a healthy body with exercise. However, high-intensity sports should begin to be replaced by lower intensity activities such as pilates, yoga, swimming, etc.

It is also important that women get adequate rest and sleep the necessary hours. To do so, going for a walk every day can help relax the body and maintain good sleep habits.

Do not abandon physical activity completely, unless it is a high-risk and recommended by your doctor.

FAQs from users

What is the status of the embryo in the third week of pregnancy?

By Jon Ander Agirregoikoa M.D. (gynecologist).

At the beginning of the third week of pregnancy there is ovulation and fertilization. The woman has not missed her period so she will not know that she is pregnant.

On the first day, at 16-20h post-fertilization, the zygote appears. On day two, the embryo has four cells, day three eight and day fourteen, at this moment it is called a morula. The fifth day follows the increase in cells and is called a blastocyst.

On the sixth-seventh day the embryo reaches the uterus and begins its adhesion to the endometrium, implantation. Normally the woman during this week does not have any symptoms.

What month is the third week of pregnancy?

By Zaira Salvador B.Sc., M.Sc. (embryologist).

The third week of gestation coincides with the first month of pregnancy, but the woman has not yet had the menstrual delay and does not know that she is pregnant.

On the other hand, the embryo has only one week of embryonic development. It has just been conceived and only consists of a few cells. It is not yet called a fetus because it has not been implanted in the uterus. It is also not possible to visualize it with an ultrasound because its size is minuscule and the gestational sac has not yet formed.

Is it normal to bleed in the third week of pregnancy?

By Zaira Salvador B.Sc., M.Sc. (embryologist).

The bleeding that some women have at the end of the third week of gestation corresponds to the implantation bleeding caused by the adhesion of the embryo in the endometrium. This bleeding is not very abundant, sometimes it is simply a stain that lasts one or two days. If it is an abundant bleeding, it could be due to an advance of the menstrual period.

How can I abort at 3 weeks of pregnancy?

By Zaira Salvador B.Sc., M.Sc. (embryologist).

Since during this third week of gestation it is still not possible to detect the pregnancy by means of a test or to know for sure that pregnancy has taken place, it is not possible to perform an abortion itself. The only way to prevent the implantation of the embryo in the uterus if there is a risk of unwanted pregnancy is through the morning-after pill.

This pill, also known as Plan B, contains levonorgestrel, a substance that causes the alteration of the endometrium and prevents pregnancy from taking place.

Is it normal to have brown discharge at 3 weeks pregnant?

By Marta Barranquero Gómez B.Sc., M.Sc. (embryologist).

No, it is not usual or common to have a brown vaginal discharge in week 3 of pregnancy. At this time, the woman may have more abundant whitish discharge, but it is rare to have brown discharge.

Can I drink alcohol in week 3 of pregnancy?

By Marta Barranquero Gómez B.Sc., M.Sc. (embryologist).

It is not advisable to drink alcohol in the third week of pregnancy or during pregnancy. Therefore, as soon as a woman learns that she is pregnant, she should stop drinking alcoholic beverages.

Drinking alcohol during pregnancy could affect the development of the baby and increase the risk of miscarriage and placental abruption.

If you want to know more about the symptoms of pregnancy during this third week of gestation, we recommend reading the following article:
Am I Pregnant? – 10 Early Clues & Symptoms Before a Missed Period.

We have discussed the importance of vitamins in the diet of the mother-to-be. You can learn more by reading this article:
Recommended Vitamins for Pregnant Women.

We make a great effort to provide you with the highest quality information.

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Ashary N, Tiwari A, Modi D. Embryo Implantation: War in Times of Love. Endocrinology. 2018 Feb 1;159(2):1188-1198 (view)

Baryiah Rehman 1, Maria Rosaria Muzio. Embryology, Week 2-3. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan.2021 May 8.

Belle M, Godefroy D, Couly G, Malone SA, Collier F, Giacobini P, Chédotal A. Tridimensional Visualization and Analysis of Early Human Development. Cell. 2017 Mar 23;169(1):161-173.e12 (view)

Danielle B. Cooper, Lily Yang. Pregnancy And Exercise. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan. 2021 Apr 26 (view)

Israr Khan, Bennett L. Leventhal. Developmental Delay. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan.2021 Aug 4 (view)

FAQs from users: 'What is the status of the embryo in the third week of pregnancy?', 'What month is the third week of pregnancy?', 'Is it normal to bleed in the third week of pregnancy?', 'How can I abort at 3 weeks of pregnancy?', 'Is it normal to have brown discharge at 3 weeks pregnant?' and 'Can I drink alcohol in week 3 of pregnancy?'.

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Authors and contributors

 Jon Ander  Agirregoikoa
Jon Ander Agirregoikoa
Graduated in medicine from the "Pays Basque" University, with a specialization in obstetrics and gynaecology. He has several years of experience in the field of assisted human reproduction and is co-director and co-founder of the ART clinics. He also combines his medical activity with teaching at the "Pays Basque" University. More information about Jon Ander Agirregoikoa
License: 014809788
 Marta Barranquero Gómez
Marta Barranquero Gómez
B.Sc., M.Sc.
Graduated in Biochemistry and Biomedical Sciences by the University of Valencia (UV) and specialized in Assisted Reproduction by the University of Alcalá de Henares (UAH) in collaboration with Ginefiv and in Clinical Genetics by the University of Alcalá de Henares (UAH). More information about Marta Barranquero Gómez
License: 3316-CV
 Zaira Salvador
Zaira Salvador
B.Sc., M.Sc.
Bachelor's Degree in Biotechnology from the Technical University of Valencia (UPV). Biotechnology Degree from the National University of Ireland en Galway (NUIG) and embryologist specializing in Assisted Reproduction, with a Master's Degree in Biotechnology of Human Reproduction from the University of Valencia (UV) and the Valencian Infertility Institute (IVI) More information about Zaira Salvador
License: 3185-CV
Adapted into english by:
 Michelle Lorraine Embleton
Michelle Lorraine Embleton
B.Sc. Ph.D.
PhD in Biochemistry, University of Bristol, UK, specialising in DNA : protein intereactions. BSc honours degree in Molecular Biology, Univerisity of Bristol. Translation and editing of scientific and medical literature.
More information about Michelle Lorraine Embleton

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