Week 4: Implantation

This week, one of the most important moments in early pregnancy occurs: implantation. The tiny blastocyst burrows into the uterine lining and attaches there. This is the start of the close partnership between you and the growing baby.

When this happens, your body begins producing hCG — the pregnancy hormone that makes a pregnancy test turn positive. Toward the end of this week, you may get your first positive test result!

For many people, this is the moment when pregnancy starts to feel real. A little line on a pregnancy test can turn your whole world upside down — in the best possible way.

Baby's Development

Implantation: The blastocyst attaches to the uterine wall, usually six to ten days after fertilization. It burrows into the thick, nutrient-rich lining
Heart tube forms: The very first cells that will become the heart begin to take shape as a primitive tube
Placenta starts: Cells that will become the placenta begin growing into the uterine wall to establish a nutrient supply and connection to the mother's blood system
Two cell layers: The embryo divides into two layers — epiblast (which will become the baby) and hypoblast (which will become support structures)
Amniotic membrane forms: The inner amniotic membrane (amnion) begins forming around the embryo
Yolk sac: A small yolk sac supplies the embryo with nutrients in the earliest weeks, before the placenta is fully developed

Implantation in Detail

Implantation is a remarkable process that lasts about two to three days. The blastocyst first attaches to the surface of the uterine lining (adhesion), then burrows between the cells (invasion).

The outer cells of the blastocyst (trophoblasts) produce enzymes that help it bore into the lining. When implantation is complete, the embryo is entirely surrounded by the uterine lining, and the first blood vessels between mother and embryo begin to form.

This process sets off hCG production (human chorionic gonadotropin). hCG signals the corpus luteum to continue producing progesterone, which is essential for maintaining the pregnancy in the first weeks.

Early Development of the Placenta

Already now, the cells that will become the placenta begin to establish themselves. Trophoblast cells grow into the uterine lining and create a network of blood vessels connecting the mother's and baby's circulatory systems. The placenta will become a remarkable organ — it functions as the baby's lungs, kidneys, and digestive system throughout the entire pregnancy, supplying the baby with oxygen and nutrients while removing waste products.

Your Body

Implantation bleeding: Some people experience a light, pink or brown discharge when the embryo attaches. This is completely normal and harmless
hCG production starts: The pregnancy hormone hCG begins to rise and can double every 48 hours in the beginning
Missed period: Toward the end of the week, your period would normally be due, but now it does not arrive — often the first sign many notice
Early signs: Some people notice tender breasts, fatigue, or a slight metallic taste in the mouth
PMS-like symptoms: It can be difficult to distinguish early pregnancy symptoms from normal PMS at this stage
Increased discharge: Some experience more discharge than usual, which is thinner and lighter in color

Early Pregnancy Symptoms vs. PMS

Many of the earliest pregnancy symptoms closely resemble PMS. Tender breasts, fatigue, mood swings, and abdominal discomfort can occur with both. Some signs more typical of early pregnancy include:

Metallic taste in the mouth
Heightened sense of smell
Small amounts of pink or brown bleeding (implantation bleeding)
A level of fatigue that feels deeper than typical PMS tiredness
More frequent urination

The only reliable way to know whether you are pregnant is to take a pregnancy test after a missed period.

Good to Know

You can soon take a pregnancy test! Most home tests are reliable from the first day after the expected period. Some sensitive tests can show a result a few days earlier, but the risk of a false negative is higher then.

If the test is negative but your period has not arrived, wait a few days and test again. hCG levels may be too low to register at the very start.

Book an appointment with your doctor or midwife when you get a positive result. The first prenatal appointment is usually around weeks 8–12. Your doctor can confirm the pregnancy and refer you to a midwife.

Use our due date calculator to estimate your expected delivery date based on your last period.

Implantation bleeding occurs in approximately 20–30 percent of all pregnancies. It can be mistaken for a light period. The difference is that implantation bleeding is usually lighter in color, short-lived (1–2 days), and does not contain clots.

Pregnancy Test — How It Works

A pregnancy test measures the level of hCG (human chorionic gonadotropin) in urine. Here is what you should know:

Timing

Morning urine is best: First urine of the morning has the highest hCG concentration
Wait for the right time: For the most reliable result, wait until the first day after the expected period
Early tests: Some tests are marketed as "early" and may give a result a few days before the expected period, but the chance of a false negative is higher

Interpreting the Result

A faint line is still positive: Even a faint line means hCG has been detected — you are pregnant
No line: Tested too early, or not pregnant. Test again in a few days
Blood test at the doctor's: Can measure the exact hCG level and is more accurate than a urine test

hCG Values in Early Pregnancy

In a normal pregnancy, hCG levels roughly double every 48 hours in the first weeks:

Week 4: approximately 5–426 mIU/ml
Week 5: approximately 18–7,340 mIU/ml
Week 6: approximately 1,080–56,500 mIU/ml

Variation is large between different pregnancies, so it is difficult to draw conclusions from a single hCG level.

First Steps After a Positive Test

When you see two lines on the pregnancy test, a lot happens at once — emotions, thoughts, and practical planning. Here is what to do:

Contact Your Doctor or Midwife

Your doctor is often the first point of contact. They can confirm the pregnancy and refer you to prenatal care. Most healthcare systems provide access to prenatal care throughout pregnancy.

Choose Your Prenatal Care Provider

Depending on your location, you may have options including:

Midwife-led care: Midwives specialize in pregnancy and birth and often provide the main prenatal care
Obstetrician-led care: May be recommended if you have a high-risk pregnancy or medical conditions
Shared care: A combination of both, working with your general practitioner

Contact your care provider early — in some areas there can be waiting lists.

Start Supplements

According to WHO and ACOG, you should take:

Folic acid: 400 micrograms daily (ideally already started)
Vitamin D: 10–15 micrograms (400–600 IU) daily throughout the year
Iodine: Discuss with your healthcare provider if you eat few dairy products and little fish

What to Expect Going Forward

First prenatal appointment around weeks 8–12
First trimester screening (nuchal translucency + blood tests) offered around weeks 11–14
Anatomy scan around weeks 18–20
Check the pregnancy timeline for a complete overview

Supplements in Pregnancy

These are the most important supplements in pregnancy:

Folic Acid — 400 micrograms daily

Folic acid is essential for the development of the neural tube. The neural tube closes as early as weeks 5–6, often before many people know they are pregnant. It is therefore recommended to take 400 micrograms daily from at least one month before conception and through the entire first trimester.

Vitamin D — 10–15 micrograms daily

Important for calcium absorption and the baby's bone and tooth development. According to the WHO, vitamin D is recommended for pregnant people throughout the year, especially in regions with limited sun exposure during winter months.

Iodine — 220 micrograms daily (WHO recommendation)

Iodine is important for thyroid function and the baby's brain development. According to WHO, many pregnant people do not get enough iodine. Good sources are dairy, eggs, and fish. Discuss supplementation with your healthcare provider.

Iron — when needed

Iron needs increase in pregnancy. Iron levels are checked at the first prenatal appointment, and supplementation is recommended if levels are low.

Who to Tell — and When

One of the big questions after a positive test is who to tell, and when.

Common Timings

Weeks 4–6 (now): Many choose to tell their partner immediately, and perhaps very close family members
Weeks 8–12: Some wait until after the first ultrasound or until the risk of miscarriage has fallen
Weeks 12–14: The most common time to share the news with friends, colleagues, and social media
Employer: In most countries, you are not required to inform your employer early, but it may be practical if nausea or fatigue affects your work

Things to Consider

There is no right or wrong time — do what feels right for you
Some choose to tell early to have support whatever the outcome
Others prefer to keep it private until they feel more secure
Talk with your partner about what you are both comfortable with

Mental Health After a Positive Test

It is completely normal to experience a wide range of feelings after a positive pregnancy test:

Overwhelm: Even if the pregnancy was planned, it can feel surreal
Worry: Thoughts like "can we handle this?" and "what if something goes wrong?" are common
Grief and joy at the same time: Especially for those who have experienced previous losses or a long wait
Uncertainty: It is normal not to feel "right" right away

Prenatal mental health matters. Your midwife or doctor will ask about this at check-ups. Do not hesitate to talk to someone if you experience persistent anxiety or low mood.

Exercise and Physical Activity

It is safe to continue with moderate exercise in early pregnancy. Aim for at least 150 minutes of moderate activity per week:

Walking: Safe and effective throughout pregnancy
Swimming: Relieves the joints and is gentle on the body
Pelvic floor exercises: Important for preparing the body for pregnancy and birth
Yoga and pilates: Build strength, flexibility, and help with stress management

Avoid contact sports and activities with a high risk of falling. Listen to your body — if something feels wrong, ease up.

For Your Partner

Be prepared for a possible positive test — it can be overwhelming and exciting at the same time
Support your partner regardless of the result, and be patient if it takes time
Plan a meaningful way to share the news together if the test is positive
Start reading about pregnancy and birth — follow the pregnancy timeline week by week to know what is coming
Be involved in contacting healthcare providers — being engaged from the start is positive

Tips for Week 4

Wait for the right time to test: For the most reliable result, wait until the first day after the expected period
Test in the morning: Morning urine has the highest hCG concentration and gives the most reliable answer
Book an appointment: When you have a positive test, contact your doctor or midwife for the first prenatal appointment
Do not be stressed by a faint line: A faint line on a test is still positive — hCG levels are just still low
Keep taking folic acid and vitamin D: These are the most important supplements in early pregnancy
Stay organized: Use our pregnancy tools to keep track of everything you should do in early pregnancy

When to Contact a Doctor

Contact your doctor or midwife if you experience:

Heavy bleeding with clots
Severe, one-sided abdominal pain (may indicate ectopic pregnancy)
Dizziness combined with bleeding
Fever over 38°C (100.4°F)
Persistent vomiting making it impossible to keep fluids down
Severe anxiety affecting daily life

Did You Know?

The pregnancy hormone hCG can double every 48 hours at the beginning of pregnancy. This is the hormone that makes a pregnancy test show a positive result.
Implantation bleeding occurs in approximately 20–30 percent of all pregnancies and can easily be mistaken for a light period. It is usually lighter and shorter than a normal period.
The fertilized egg has traveled about 15 centimeters through the fallopian tube to reach the uterus — a journey that takes about 4–5 days.
Already now, cells that will become the placenta are growing into the uterine wall. The placenta is a completely new organ that your body creates specifically for this pregnancy!
hCG is the hormone that causes nausea for many pregnant people. Higher hCG levels are associated with stronger nausea, which explains why nausea is often worst between weeks 6 and 12 when hCG levels are at their peak.
The very first pregnancy test was developed in the 1920s and involved injecting the woman's urine into a frog or rabbit. Modern home tests became available in the 1970s.
The placenta is the only organ the body creates completely new for each pregnancy. It weighs about 500 grams at birth and has a surface area of approximately 14 square meters.
Progesterone production from the corpus luteum in the ovary is vital to maintaining the pregnancy in the first weeks. Later, the placenta takes over this task, usually around weeks 10–12.

Frequently Asked Questions About Week 4

What is the difference between implantation bleeding and menstruation?

Implantation bleeding is usually lighter in color (pink or light brown), shorter in duration (1–2 days), and lighter than a normal period. It does not contain clots and is rarely accompanied by severe cramps. If you are unsure, wait a few days and take a pregnancy test.

How early can a pregnancy test show positive?

The most sensitive tests can show positive as early as 8–10 days after fertilization, but for the most reliable result you should wait until the first day after the expected period. Testing too early risks a false negative because hCG levels may still be too low.

Can a faint line on a test mean something other than pregnant?

A faint line on a pregnancy test generally means you are pregnant, but that hCG levels are still low. Test again in two to three days — the line should be darker because hCG doubles quickly early on. False positives are extremely rare.

Should I start with a midwife or a GP for prenatal care?

This depends on your country and local healthcare system. In many countries, you can choose midwife-led care, GP care, or a combination. Midwives specialize in pregnancy and birth and often provide excellent continuity of care. Contact your preferred provider early, as availability can vary.

Is it normal to feel anxious after a positive test?

Yes, it is completely normal to have mixed feelings. Joy, anxiety, uncertainty, and overwhelm can all come at the same time. Many people feel a combination of great happiness and nerves about everything that is coming. Talk to your partner, a friend, or your midwife if you need someone to share your thoughts with. Persistent anxiety should be raised with your doctor.

What is a chemical pregnancy?

A chemical pregnancy means a fertilized egg implants in the uterus and hCG production starts (positive test), but the pregnancy stops very early — often before week 5–6. It is experienced as a late period. Chemical pregnancies are common and do not mean something is wrong with you. Many people experience this without knowing it because they did not test early enough.

Can I keep exercising as before?

It is generally safe to continue with moderate exercise in pregnancy. You do not need to dramatically change your exercise routine — just avoid contact sports and activities with a high risk of falling. Listen to your body. Pelvic floor exercises are recommended for all pregnant people from the start.

What does my hCG level mean?

hCG levels vary enormously between different pregnancies. In week 4, hCG can be between 5 and 426 mIU/ml. The most important thing is that the level rises steadily — it should roughly double every 48 hours in the first weeks. A single hCG value says little on its own. Your doctor can take two blood tests 48 hours apart to check that the rise is normal.

How early can an ultrasound show something?

Usually, a gestational sac can be seen on ultrasound from week 5, and a heartbeat from weeks 6–7. This is why routine ultrasound is typically not done before weeks 7–8 at the earliest. Having an ultrasound too early can cause unnecessary worry because it is simply too early to see anything.

Can I fly in early pregnancy?

Air travel is safe for most pregnancies in the first trimester. There is no increased risk from flying in early pregnancy. Airport security scanners are also not harmful to the embryo. On longer flights, drink enough fluids, move around regularly, and consider wearing compression stockings to prevent blood clots.

Next week: The heart starts beating for the very first time!