Week 3: Fertilization

Something truly remarkable is happening now! If a sperm cell has made its way to the egg in the fallopian tube, fertilization occurs. The moment the two cells fuse together, a completely new cell is created with a unique set of DNA — a combination of you and your partner.

Sex, eye color, hair color, and thousands of other traits are already determined. The tiny cluster of cells begins its journey toward the uterus — a trip that takes about four to five days. You can already use a due date calculator to estimate your expected delivery date.

Out of the millions of sperm that started the journey, only a single one has managed to penetrate the egg's protective layer. It truly is a small miracle in progress.

Baby's Development

Fertilization: The sperm penetrates the outer layer of the egg (zona pellucida), and the two nuclei fuse into a zygote with 46 chromosomes — 23 from the mother and 23 from the father
Sex is determined: Whether the baby will be a boy or girl is decided right now — determined by the sperm's sex chromosome (X gives a girl, Y gives a boy)
Cell division begins: The zygote starts dividing within hours — from one cell to two, four, eight, and so on
Morula forms: After three days, the cell cluster has about 16 cells and is called a morula (Latin for "little mulberry")
Blastocyst forms: After four to five days, the cluster has become a blastocyst with over 100 cells, ready to attach to the uterus

From Fertilization to Blastocyst

Fertilization usually takes place in the outer third of the fallopian tube. The moment a sperm penetrates the egg, the egg's outer layer (zona pellucida) changes instantly so that no more sperm can enter. This is called the "zona reaction" and ensures the egg is fertilized by only one sperm.

The two nuclei approach each other and fuse — and a new life is created. The fertilized egg (zygote) starts dividing immediately while slowly being transported through the fallopian tube toward the uterus by cilia and muscle contractions.

After about three days, it is a compact cluster of cells called a morula. This develops further into a blastocyst, which consists of an outer cell group (trophoblast, which will become the placenta) and an inner cell mass (embryoblast, which will become the baby). This entire journey from fallopian tube to uterus takes four to five days.

Genetics and Inheritance

At the moment of fertilization, the baby's genetic blueprint is set. The zygote contains 46 chromosomes — 23 from the mother and 23 from the father. These chromosomes carry approximately 20,000–25,000 genes that determine everything from eye color and hair type to blood type and growth patterns.

The sex chromosomes are particularly interesting: the mother's egg always carries an X chromosome, while the father's sperm can carry either X or Y. The combination XX gives a girl, while XY gives a boy. It is therefore the father's sperm that determines the sex — but no one can influence this, it is entirely random.

Your Body

You likely notice nothing: It is too early for symptoms. The body has not yet started producing the pregnancy hormone hCG
Progesterone rises: The corpus luteum in the ovary produces progesterone, which keeps the uterine lining thick and ready for implantation
Too early for a test: A pregnancy test will not show a positive result yet — it takes a few more days
The immune system adjusts: Your body begins subtle changes to avoid rejecting the fertilized egg
The uterus is ready: The uterine lining is thick and nutrient-rich, ready to receive the blastocyst when it arrives
Body temperature stays elevated: After ovulation, temperature remains elevated due to progesterone — an early sign for those measuring basal body temperature

The Immune System and Pregnancy

Something fascinating is happening in your body right now: the immune system begins to adjust to accept the fertilized egg, which contains DNA from another person. The body produces special proteins and immune cells that protect the embryo from being rejected, while the immune system continues to protect you from infections.

This balance is one of nature's most impressive mechanisms. The uterine lining contains specialized immune cells (uterine NK cells) that help the embryo attach and support the development of the placenta. Without this adaptation, the body would treat the embryo as a foreign intruder.

Good to Know

Even though you do not know whether fertilization has occurred, it is important to continue with healthy habits. Folic acid is especially important right now because the neural tube (which becomes the brain and spinal cord) begins to form very early in pregnancy.

Avoid alcohol, smoking, and unpasteurized dairy products. It is safe to continue with moderate activity in early pregnancy. You do not need to change your exercise habits yet.

It is completely normal to notice nothing at all during this week. Many people do not know they are pregnant until several weeks later. Simply taking care of yourself is enough.

Also be aware that some foods should be avoided: raw fish (sushi with raw fish), raw meat, unpasteurized cheese, and large amounts of liver (due to high vitamin A). Liver pâté in normal amounts (1–2 slices per day) is safe and is a good source of iron.

Even if you had a glass of wine or took a medication before knowing you were pregnant, you generally do not need to worry. In the very first days after fertilization, the egg has its own nutrient supply and is well protected. Talk to your doctor if you are concerned.

Food and Diet in Early Pregnancy

You do not need to eat for two, but the nutritional quality of what you eat matters:

What to Eat

Vegetables and fruit: At least 5 portions daily — rich in vitamins, minerals, and fiber
Whole grains: Oats, whole wheat bread, brown rice — provide lasting energy and fiber
Lean meat and fish: Important protein sources. Aim for fish 2–3 times per week
Beans and lentils: Good sources of protein, iron, and folate
Dairy products: Pasteurized milk and cheese provide calcium and iodine

What to Avoid

Raw fish and raw meat: Risk of parasites and bacteria such as listeria and toxoplasmosis
Unpasteurized cheese and milk: Risk of listeria bacteria
Large amounts of liver: Too much vitamin A. Liver pâté in moderate amounts as a spread is safe
Alcohol: No safe amount according to WHO and CDC
Smoking: Increases risk of low birth weight, premature birth, and complications
Raw eggs: Risk of salmonella. Cooked eggs are safe

Food Hygiene

Good food hygiene is especially important to prevent toxoplasmosis and listeriosis:

Wash fruit and vegetables thoroughly
Keep raw and cooked meat separate
Wash hands thoroughly after handling raw meat
Wear gloves when gardening (cat feces may contain toxoplasmosis)
Ensure meat is fully cooked through

The Two-Week Wait

The period between ovulation/fertilization and the earliest possible pregnancy test is often called the "two-week wait." It is a period many find frustrating — you do not know whether fertilization has happened, and it is too early to test.

Tips for Getting Through the Wait

Stay occupied: Work, hobbies, and social activities help the time pass
Avoid symptom-hunting: It is tempting to interpret every little change in your body, but most early symptoms are identical to PMS
Do not test too early: A negative test before the expected period can be a false negative and creates unnecessary disappointment
Take care of yourself: Eat well, sleep enough, stay active, and do things that bring you joy
Talk with your partner: Share thoughts and feelings instead of searching the internet for symptoms

What Happens in the Body During the Wait

Even though you cannot feel it, a lot is happening:

Days 1–3 after fertilization: The zygote divides and travels through the fallopian tube
Days 4–5: The blastocyst arrives in the uterus
Days 6–10: Implantation occurs — the blastocyst attaches to the uterine lining
Days 10–14: hCG begins to rise to measurable levels

Only from days 10–12 after fertilization can a very sensitive test possibly show a positive result.

Substances, Medications, and Environmental Toxins

In the period around fertilization and implantation, the embryo is most vulnerable to external influences:

Substances

Alcohol: There is no safe amount according to WHO and CDC. Cut it out entirely
Smoking: Nicotine can affect both fertilization and early development. Stop completely
Cannabis and other drugs: Can affect hormones and fetal development. Strongly discouraged

Medications

Some common medications are unsafe in pregnancy — talk to your doctor
Never stop prescribed medications without consulting your doctor
Over-the-counter medications such as ibuprofen should be avoided — use paracetamol (acetaminophen) as needed
Herbal and natural remedies are not necessarily safe — ask your pharmacist

Environmental Toxins

Limit intake of large predatory fish (tuna, swordfish) due to mercury
Avoid contact with pesticides and strong chemicals
Wear gloves when gardening (toxoplasmosis from soil)
Have someone else empty the cat litter box

For Your Partner

Be patient — it is too early to test, and the wait can feel long
Continue supporting healthy habits such as good nutrition and moderate activity together
Read up on what is happening in the body — it makes it easier to understand and support each other through pregnancy
Be mindful of your own health too — diet, exercise, and stress levels affect everyone in the family
Talk about expectations and feelings — it is natural to have many thoughts during this period

Tips for Week 3

Keep taking folic acid: Folic acid is critical for the neural tube's development, which begins very early
Be patient: It is too early to take a pregnancy test — wait until after the expected period
Avoid harmful substances: Alcohol, smoking, and certain medications should be avoided from now on
Check your diet: Avoid raw fish, raw meat, and unpasteurized dairy products
Wash fruit and vegetables: Good food hygiene prevents toxoplasmosis and other infections
Stay active: Moderate exercise is safe and recommended in pregnancy

When to Contact a Doctor

Contact your doctor or midwife if you experience:

Severe abdominal pain on one side (can be a sign of ectopic pregnancy)
Unusually heavy bleeding
Fever over 38°C (100.4°F)
Concerns about medications you are taking and their effect on a possible pregnancy
Dizziness or fainting combined with abdominal pain

Did You Know?

Of the 200–300 million sperm that start the journey, only a few hundred reach the egg — and only one fertilizes it. It is a competition of enormous selection!
The baby's sex is determined in the second the sperm penetrates the egg. If the sperm carries a Y chromosome it will be a boy, and an X chromosome gives a girl. The mother's egg always carries an X chromosome.
Within the first 24 hours after fertilization, the fertilized egg divides from one cell to two. After four days there are already over 100 cells!
Fraternal twins arise when two eggs are fertilized by two different sperm. Identical twins arise when one fertilized egg splits into two separate embryos during the first days.
Sperm must swim about 15–18 centimeters to reach the egg. Relative to body size, this is an enormous distance.
The blastocyst that attaches to the uterus is only about 0.1–0.2 mm in size, but already contains over 100 cells organized into different layers with different roles.
Every human being starts life as a single cell — the zygote. From this one cell, a complete human being develops with approximately 37 trillion cells.

Frequently Asked Questions About Week 3

Can I feel that fertilization has happened?

No, there is no way to feel fertilization itself. Some people report a "feeling" or slight cramp a few days later (at implantation in week 4), but the actual fusion of egg and sperm is completely imperceptible. It is too early for symptoms because the body has not yet started producing the pregnancy hormone hCG.

Is it dangerous that I drank alcohol before knowing I was pregnant?

Many people worry about this, but you generally do not need to be concerned. In the very first days after fertilization, the fertilized egg has its own nutrition and is not yet connected to your blood supply. Stopping alcohol as soon as you know about the pregnancy is generally fine. Talk to your doctor if you are worried.

Can I take a pregnancy test in week 3?

It is too early for a home test in week 3. The pregnancy hormone hCG does not start being produced until the fertilized egg attaches to the uterus (implantation), which usually happens toward the end of week 3 or in week 4. Most tests give reliable results from the first day after the expected period.

What happens if the egg is not fertilized?

If the egg is not fertilized within 12–24 hours after ovulation, it breaks down and is absorbed by the body. The uterine lining is shed as a normal period about two weeks after ovulation, and a new cycle begins.

Should I stop taking X-rays if I might be pregnant?

If you suspect you might be pregnant, you should inform healthcare staff before any X-ray procedures. Routine dental or limb X-rays deliver a very low dose of radiation and are generally considered safe, but X-rays of the abdomen and pelvis should be avoided or postponed where possible in early pregnancy. Talk to your doctor if you are unsure.

What is an ectopic pregnancy?

An ectopic pregnancy means the fertilized egg implants outside the uterus, usually in the fallopian tube. It occurs in approximately 1–2 percent of all pregnancies. Symptoms include severe pain on one side of the abdomen, bleeding, and dizziness. An ectopic pregnancy requires prompt medical treatment — contact a doctor immediately if you suspect one.

Can stress in this period affect fertilization?

Brief, normal stress does not usually affect the chance of fertilization. Prolonged, chronic stress can, however, affect hormonal balance and ovulation. Physical activity, mindfulness, and getting enough sleep can all contribute positively.

Can IVF help if it is taking a long time?

Assisted reproduction is available to couples who have not conceived after a sufficient period of trying. IVF success rates depend on age and other factors, and most healthcare systems have criteria for eligibility. Typically, an assessment begins after 12 months of trying (6 months for people over 35). Speak with your doctor for a referral to a fertility specialist.

What are twins and how do they occur?

Fraternal twins arise when two eggs are fertilized by two different sperm in the same cycle. They are genetically different, like ordinary siblings. Identical twins arise when one fertilized egg splits into two during the first days. They have identical DNA. The chance of twins increases with the mother's age and with fertility treatment.

Should I stop coffee entirely now?

You do not need to stop coffee, but you should limit your intake. According to WHO and major health bodies, a maximum of 200 mg of caffeine daily is recommended during pregnancy, equivalent to about 2 cups of filtered coffee. Too much caffeine can affect the developing baby. You can switch to decaf for some cups, or switch to tea, which has a lower caffeine content.

Next week: The fertilized egg attaches to the uterine wall, and hCG production begins!