You are now 5 weeks pregnant and have entered what is called the embryonic period of development; during this time, your baby’s major organ systems and structures will develop.

At this stage in development, the embryo’s cells differentiate or multiply and will begin to take on specific functions.

This MNT Knowledge Center feature is part of a series of articles on pregnancy. It provides a summary of each stage of pregnancy, what to expect, and insights into how your baby is developing.

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.

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While all your baby’s organs are developing, the primitive placenta and umbilical cord are functioning to provide your baby with the nourishment and oxygen necessary for survival.

You will likely have missed your period, so you should take a pregnancy test. You may also be noticing some early symptoms of pregnancy, including:

  • mood swings
  • breast tenderness – often a general aching
  • exhaustion – eating every few hours can help keep blood sugar levels up to combat sleepiness
  • waves of nausea – famously called morning sickness, it can occur at any time of day
  • increased urinary frequency – because your kidneys are expanding
  • cramps – due to the uterus growing and ligaments stretching
  • spotting – this can be worrying but is normal

At this stage, you are unlikely to have any external signs that might give your condition away. It is worth noting that some women do not experience any symptoms at all at this stage, and this should not be a cause for concern.

Now might be a good time to start an exercise routine to help strengthen your muscles for the increased workload they are soon to undertake. Also, women who are fitter before birth tend to bounce back quicker after delivery.

Various hormones are circulating throughout your body to maintain and support your pregnancy.

  • Human Chorionic Gonadotropin (hCG): This hormone is vital to the health of your pregnancy as it supports the first means of nourishment to your baby, the corpus luteum. At the end of the first trimester, the placenta will be fully functional. Additionally, hCG is responsible for maintaining adequate levels of progesterone.
  • Progesterone: This hormone has many roles. Not only is it responsible for maintaining placental function, but it is also responsible for inhibiting uterine contractions and stimulating breast tissue growth.
  • Estrogen: This hormone is produced to maintain progesterone and hCG levels.

Your baby is growing at a rapid pace with several cell types and organs starting to develop, including the blood cells, kidney cells, nerve cells, brain, spinal cord, heart, and gastrointestinal tract.

By now, the embryo consists of three layers:

  • Ectoderm: The neural tube, responsible for forming the brain, spinal cord, nerves, and backbone is developing from the ectoderm, which will also go on to form the skin, hair, nails, mammary and sweat glands, and tooth enamel.
  • Mesoderm: Your baby’s heart and circulatory system will form from the mesoderm, which will also form muscles, cartilage, bone, and subcutaneous tissues. Your baby’s heart, which is now divided into two “tubes,” may be visible beating on an ultrasound.
  • Endoderm: The lungs, intestines, urinary system, thyroid, liver, and pancreas form from the endoderm.

While these organs are developing, the primitive placenta and umbilical cord are functioning and are providing your baby with the nourishment and oxygen necessary for survival.

Your growing baby is vulnerable to the effects of various elements including the use of certain medications, illegal drugs, excessive alcohol consumption, and infections, such as rubella.

Your baby is now the size of a sesame seed and looks more like a tadpole than a baby.

In your first prenatal visit, your health provider will advise you about any changes you should make to keep you and your baby safe.

It is important, especially during the first 12 weeks, to avoid certain infections.

Flu shot

The flu can be more serious when you are pregnant, and it can develop to become pneumonia, putting you and your developing baby at risk. A fever, for example, has been linked to neural tube defects in the developing child.

The Centers for Disease Control and Prevention (CDC) call the flu jab “the first and most important step” in preventing the disease. They recommend it for everyone during pregnancy.

If you develop flu symptoms, you should seek medical help at once. Antiviral treatment is effective, but it should start within 48 hours.

Symptoms of flu include:

  • breathing difficulties
  • pain in the chest or abdomen
  • dizziness and confusion
  • severe vomiting
  • a high fever

The flu jab has a long safety record and is a safe option compared with the problems that may result from flu complications.

Other issues

If you have questions regarding your pregnancy, be sure to contact your healthcare provider.

Call your doctor if you are experiencing symptoms of an ectopic pregnancy or miscarriage, such as vaginal bleeding or passage of tissue, leaking vaginal fluid, feeling faint or dizzy, low blood pressure, rectal pressure, shoulder pain, severe pelvic pain, or cramping.