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Pregnancy health centre

4 weeks pregnant

WebMD Medical Reference
Medically Reviewed by Dr Rob Hicks

egg in tube

How is your baby growing at 4 weeks?

In week 4 of pregnancy the blastocyst inner cells are now forming in two distinct layers - the epiblast will become the embryo and amniotic cavity and the hypoblast will form into the yolk sac to nourish the developing baby.

The embryo will be a mass of cells of up to 1mm long. The cells are now in the process of developing into three distinct layers that will form organs and tissues.

  • The outside layer is the ectoderm and will form into the neural tube, from which the brain, spinal cord and nerves will develop, as well as the lenses of the eyes, skin, nails, tooth enamel, nose and parts of the ears.
  • The middle layer is the mesoderm and will become the heart and circulatory system, as well as muscles and bones.
  • The inner layer is the endoderm and will eventually form into the lungs, digestive tract and urinary system, as well as the tongue and tonsils.

At the moment the yolk sac is providing the embryo with nutrients until the placenta can take over, and a fluid inside the sac – known as amniotic fluid – is surrounding and protecting the embryo. Implantation continues, with finger-like chorionic villi developing from the outer layer of the yolk sac to anchor the embryo to the womb and form the beginnings of a placenta. The placenta will develop to provide the embryo with oxygen and nourishment throughout the pregnancy. The chorionic villi will also form a stalk between the placenta and embryo that will later make up part of the umbilical cord.

How are you changing in week 4?

Once the embryo is implanted, it triggers the production of human chorionic gonadotropin (hCG), a hormone that helps your body to maintain the lining of the womb. This hormone will also send a signal to your ovaries to stop them releasing eggs, and your monthly menstrual cycles will stop for the duration of your pregnancy.

It's likely that you would normally be expecting your period this week. The hormone progesterone, still being produced by the corpus luteum, can make you fill nauseous and fatigued and make your breasts ache or tingle, but these early signs of pregnancy are also similar to those of premenstrual syndrome (PMS). A missed period may be the first sign that you are pregnant.

What you need to know in week 4

If you want to find out if you are pregnant, home pregnancy test kits that can give you an immediate answer are available at most chemists. These are simple to perform, using a sample of urine to determine if you are pregnant. If you follow directions carefully, the results are generally reliable. However, sometimes there can be a "false negative", when there's not enough hCG hormone in your urine. Taking the test in the morning, when this hormone is more abundant, will give the best results.

If you get a false negative and your period still has not started, repeat the test a week later.

Next: 5 weeks pregnant

Additional pregnancy week by week pages:
6 weeks pregnant 7 weeks pregnant 8 weeks pregnant 9 weeks pregnant

Reviewed on January 16, 2017

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