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During week 4 of pregnancy, a person might notice some very light bleeding or spotting. This is known as implantation bleeding. This can be caused by the embryo planting itself in the wall of the womb.

This is one of a series of articles that will help you understand the changes that will be taking place week by week.

Whether you are feeling excited, worried, or confused, the more you learn about what is happening to you and your baby at each stage of your pregnancy, the better prepared you will be to embrace the changes.

Take a look at some other articles in the series:

First trimester: fertilization, implantation, week 5, week 6, week 7, week 8, week 9, week 10, week 11, week 12.

Second trimester:

week 13, week 14, week 15, week 16, week 17, week 18, week 19, week 20, week 21, week 22, week 23, week 24, week 25, week 26

Pregnancy testShare on Pinterest
A pregnancy test detects levels of hCG in the urine.

The first symptom of pregnancy is usually a missed period. If you have missed a period, you may have taken a pregnancy test already, or maybe you are planning to take one.

A pregnancy test can tell you whether or not you are pregnant by measuring levels of a hormone known as human chorionic gonadotrophin (hCG).

hCG starts to be released after the fertilized egg is implanted.

The majority of tests are most reliable the day after the period was due, but some can detect hCG, a few days before.

Other symptoms that may be starting to show include breast tenderness, headaches, lower back pain, and nausea.

What is happening?

Missing a period may be the only symptom of pregnancy you have noticed so far, but other changes will be taking place that you cannot see.

When you are not pregnant, you menstruate regularly. Every month, an egg is released, and the inner lining of the uterus thickens in preparation for pregnancy. If conception does not take place, the egg and the lining pass out through the vagina. This is menstruation.

After the egg is fertilized, a structure known as a blastocyst forms. This structure contains a mass of cells, surrounded by a protective outer shell. It will soon become an embryo.

At 4 weeks, the blastocyst has made a 6-day trip from the fallopian tubes to the womb. Here, it begins to burrow or implant into the wall of the uterus. It will take its nutrition from the mother’s blood.

Meanwhile, the amniotic sac is forming, along with the yolk sac and the placenta.

Spotting or bleeding

Approximately 6 to 12 days after the embryo is conceived, some people experience mild vaginal spotting with or without mild cramping. This is known as implantation bleeding and may be mistaken for a menstrual period.

This bleeding will generally resolve on its own, and it can be an early sign of pregnancy. However, contact your doctor if the bleeding is heavy, foul-smelling, or accompanied by chills, fever, or cramps.

Bleeding during pregnancy can be caused by factors other than implantation and may be a result of sexual activity, an ectopic pregnancy, or miscarriage.

Some types of sexual activity can cause bleeding at any time during pregnancy. Hormonal changes increase the blood flow to the cervix, making it more likely to bleed if sexual activity irritates the area.

It is important not to use tampons for bleeding during pregnancy as they increase the risk of infection.

Using sanitary pads to keep track of and measure bleeding is a safer method. If you are saturating one sanitary pad in 2 hours or less, contact your doctor.

For women who usually use tampons, here is a link for purchasing sanitary pads online. This will take you to an external site.

Ectopic pregnancy

An ectopic pregnancy happens when the embryo has implanted outside the uterus. It is considered a medical emergency. The pregnancy will not continue.

Seek medical attention immediately if you are experiencing bleeding with pain and cramps.


Loss of pregnancy is a common occurrence, especially in the early stages. If it happens before week 20, it is called a miscarriage. Between 10 and 25 percent of clinically recognized pregnancies end with a miscarriage.

It usually happens because of a chromosomal problem or a medical issue. Call your doctor if you experience bleeding or pain.

This can be a distressing time. Help is available through support groups such as Mend.

Throughout your pregnancy, you will have variations in certain hormones, which contribute to many of pregnancy’s symptoms. Listed below are some of the major contributors at this stage:

Human chorionic gonadotropin

Following implantation of the fertilized egg, your body begins to secrete the hormone hCG. This is measured in urine and blood to detect pregnancy.

HCG is responsible for regulating estrogen and progesterone and contributes to the need to urinate more frequently.


Progesterone is initially produced by the corpus luteum, a temporary structure within the ovaries. Levels rise throughout pregnancy and continue to do so until the birth of your baby.

In early pregnancy, progesterone is responsible for increasing uterine blood flow, establishing the placenta, and stimulating the growth and nutrient production of the endometrium (lining of the uterus). Also, progesterone plays a vital role in fetal development, preventing premature labor and lactation, as well as strengthening the muscles of the pelvic wall to prepare your body for labor.

In addition to progesterone, the placenta secretes vital hormones during pregnancy such as:

Human placental lactogen

Human placental lactogen is believed to play a part in breast changes that are necessary for lactation following the birth of the baby. It also plays a role in increasing nutrient levels in your blood, which is vital for embryo growth and development.

Corticotrophin-releasing hormone

Corticotrophin-releasing hormone is responsible for determining how long you will be pregnant and for embryo growth and development.

Later in pregnancy, the rise in both corticotrophin-releasing hormone and cortisol complete fetal organ development and provide the mother with a surge of cortisol. This surge has been linked with maternal attentiveness, increasing the mother-baby bond.


Estrogen is responsible for embryo and fetal organ development, placental growth and function, and preparing the breasts for lactation.

Additionally, estrogen is needed for the regulation of other hormones produced during pregnancy.

Because of the rise in progesterone and estrogen, you may experience some pregnancy symptoms such as mood swings and morning sickness.

Three layers are developing in the blastocyst: the ectoderm, the mesoderm, and the endoderm. These will eventually form the organs and tissues of the newborn infant.

The following body parts and organs will develop from the ectoderm layer during week four:

  • the nervous system and brain
  • the outermost layer of the skin
  • the hair
  • the lens of the eye
  • pigment cells
  • the nails
  • mammary and sweat glands
  • tooth enamel

From the mesoderm layer, a range of other organs is created, including:

  • the heart
  • lymph cells
  • sex organs
  • the skeleton and skeletal muscles
  • connective tissues
  • the urogenital system
  • the dermis of the skin
  • the kidneys and spleen

The endoderm layer will develop into:

  • the lungs
  • the liver
  • the pancreas
  • the pharynx
  • the stomach
  • the urinary bladder
  • the thyroid and parathyroid
  • the intestines
  • the lining of the urethra

Right now, your baby is still the size of a poppy seed and measures around 0.078 inches.

If a home pregnancy test has produced a positive result, here are some steps you can take next.

Calculate your due date

Nobody knows exactly when their baby will arrive, but if you know the date of the first day of your last menstrual period (LMP), you can use an online calculator to find out approximately when the due date will be.

See your doctor

You should contact your health provider an arrange your first prenatal visit.

Depending on when you find out you are pregnant, this may not happen until 8 weeks after your LMP.

The doctor will ask about your:

  • mental and physical health
  • height, weight, and blood pressure
  • any medications being taken, including birth control methods used
  • personal and family medical history, including allergies
  • any previous pregnancies, including losses or terminations

It is important to attend all appointments, even if this is not your first pregnancy, as each one is different.

Following a healthful lifestyle during pregnancy is good for you and your growing baby.

Here are a few tips for keeping well, but there are many more:

Avoid smoking: If you smoke, give up now. Avoid places where other people smoke, as second-hand smoke can harm your baby too.

Avoid alcohol and drugs: No alcoholic drink or recreational drug is safe during pregnancy. Children born to mothers who drink during pregnancy can have developmental and behavioral problems.

These changes must start as soon as you know or suspect you are pregnant. The first three months of pregnancy is the most vulnerable time to damage from drugs or alcohol.

Eat healthful food: Fish is good, but not shark and a number of other species, as they can contain high levels of mercury. Eat plenty of fruit, vegetables, and wholefoods to get a good supply of vitamins and other nutrients.

Vitamin: Ask your health provider which supplement to take. You should normally take one that includes folic acid.

Medicines and supplements: If you take regular supplements or medication, whether prescribed or over-the-counter, check with your health provider that they are safe to use.

Exercise: If you train already, check with your health provider that your program is safe to continue. If not, start doing moderate exercise, say, walking half an hour a day or swimming.

See your health provider about any concerns you may have.