4 Weeks Pregnant - Fetal Development

Your pregnancy journey begins

At this stage your baby is now called an embryo and is already growing rapidly. The placenta is formed and will nourish your baby for the remainder of your pregnancy by passing nutrients to them through the umbilical cord.

Your baby's development

Baby as big as a poppy seed

Length 2 mm Weight < 1 g

Your baby's development

During this period, your baby’s major organs begin to form. Their brain, liver, kidneys, pancreas, thymus and spleen are all taking shape and will eventually take on the role of sustaining them for life.

Your baby grows through a process called imprinting, which involves cells dividing and then subdividing, millions and billions of times. Every cell contains a genetic marker, or epigenetic switch. These switches are highly adaptable to their environment, switching on or off to make the cell more or less active. For example, all tissues contain the insulin gene; however the only place it ‘turns on’ is the pancreas.

Your body

Your body is already changing. Your progesterone and human chorionic gonadotropin (hCG) levels increase significantly this week. Your hCG may now be high enough to produce a positive result on a home pregnancy test. However, not all pregnancies can be detected this early.

Your changing hormones cause the lining of your uterus to build up. This, together with the fact that your uterus is already changing in size, can lead to light cramping. The sensation can feel like mild premenstrual cramps.

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Please Note: These pregnancy week-by-week articles, are meant to give you a general sense of the growth and development going on within you. However, it’s important to remember that every baby develops differently, even during pregnancy.

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