Embryo and Placenta: How Do They Develop?

What starts off as fertilized egg about 0.1 mm (0.004 in) across emerges after 9 months in your belly as a newborn clocking in at about 3400 g (7.5 lb). It seems intuitive enough, but when you sit down and think about, it you can’t help but wonder: how in the heck does that even happen!? Let’s make quick run through of how your pregnancy progresses from a tiny embryo to a full-term baby, and what role the placenta plays in getting them there.

The embryo and placenta: From implantation to fetus

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After an egg and a sperm cell combine to form a zygote, the fertilized cell divides into a collection of cells called a blastocyst. The fertilized bundle of cells then implants into the endometrium lining of the uterus where it begins to develop into an embryo.

Put simply, an embryo is the starting point of a baby. It is referred to as an “embryo” until the end of Week 10, when bone structures have begun to develop. At Week 10, the average human embryo measures about 3 cm (1.2 in) long and weighs around 4 g (0.14 oz) – roughly 300 times bigger than the fertilized egg cell it started off as! At this stage, the basic structures of the circulatory system, facial features, limbs and brain have developed. After this point, the developing baby is officially called a “fetus”.

The placenta supports the developing embryo

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The placenta is a temporary organ that develops during pregnancy in order to transfer the nutrients and oxygen the developing baby needs to grow. It also acts as a barrier between maternal and fetal blood, so that the fetus has a layer of protection from infections and harmful substances in the mother’s bloodstream (though exceptions like the rubella virus and alcohol can make it across).

Once the fertilized egg implants into the endometrium, the mother’s body releases a surge of the pregnancy hormone hCG (human chorionic gonadotropin). At the same time, a section of the endometrial lining begins to thicken, and the chorionic villi come into contact with the mother’s blood, so nutrients in the blood can nourish the developing baby. In other words, the placenta develops when two structures meet and combine: the chorion from the baby’s side, and the endometrium from Mom.

The placenta continues to develop until around Week 15 of pregnancy – around the beginning of the second trimester, when symptoms like morning sickness often stabilize and the risk of miscarriage decreases. Once it’s fully developed, the average placenta measures about 22 cm (9 in) across and 2 to 3 cm (0.8 to 1.2 in) thick, and weighs about 500 g (1 lb).

Keeping the placenta healthy for embryo growth

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The health of the mother has a big influence on the health of the placenta. If Mom’s in bad shape, then it can be hard for the placenta to transfer the nutrients the developing baby needs. In particular, high blood pressure can result in decreased blood flow to the placenta – which could mean your baby doesn’t get enough oxygen or nutrients. Ultimately, this can lead to complications like low birth weight, premature birth or even preeclampsia and placental abruption (when the placenta breaks off from the womb wall too early).

Factors such as obesity or taking in too much salt can increase the risk of high blood pressure. Make sure you watch out for symptoms like swelling or other signs of hypertension – that means it’s time to get to a doctor right away.

With the placenta complete, your baby’s development speeds up

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Once the placenta has finished developing at around the start of the second trimester, the developing fetus is able to receive plenty of oxygen and nutrients from the mother’s body. After this point, your baby undergoes a big-time growth spurt. You may also find that your morning sickness symptoms settle down as your hormone levels begin to change. It’s a big milestone in a pregnancy, so look forward to it while you ride out the first trimester!