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When Does the Placenta Form?

2 years ago4 min read
Last updated: Jan 20 2023

As your baby grows, your placenta is nourishing and protecting them. But what is a placenta exactly, and how does it work? And when does the placenta form?
We’ll get to that. But first, a quick introduction:

When Does the Placenta Form

Put simply, a placenta is a “temporary organ” that you carry in your uterus during pregnancy, which aids the growth and development of your baby by giving them the oxygen and nutrients they need – while removing waste products from their system.

The placenta can be attached to the wall of your uterus near your spine, which is called a posterior placenta. It might be attached to the wall at the front, near your belly, called an anterior placenta. Or it might be a fundal placenta, attached to the top of the uterus, and even a lateral placenta attached to the left or right side of the uterus.

You can also have a low-lying placenta (placenta previa), which can partially or fully cover the cervix. This carries extra risks, and might mean you’ll need to have a caesarean section.

So, when does the placenta start to form?

When a fertilized egg reaches the uterus from the fallopian tube, some of the cells start to form the placenta, while the rest go on to form the fetus. It is at this point the fertilized egg, or embryo, implants in the lining of the womb, which is about four to six weeks into your pregnancy. When does an embryo become a fetus? From around 10 weeks pregnant, the embryo is officially a fetus, and the placenta is formed and continues to develop.

How long does it take for the placenta to develop?

The placenta continues to develop throughout your term. By 12 weeks of pregnancy, the placenta has all it needs to support your baby. By 20 weeks, the placenta is fully-formed and by 34 weeks the placenta is considered mature. It continues growing just as your baby does, and when you deliver the placenta (after your baby is born), it can weigh about a pound.

When does the placenta take over?

In the first trimester, the main source of hormones and nourishment for your baby is provided by the follicle in the ovary that the egg came from (also known as the corpus luteum). And while every pregnancy is unique, it is thought that usually the placenta takes over from the corpus luteum and becomes the main source of nourishment for your baby when you’re between 8 to 12 weeks pregnant.

From the second trimester onwards, the placenta performs some important jobs during your pregnancy. It provides water, oxygen, antibodies and nutrients to your baby, taken from your bloodstream as it flows through the placenta. It then feeds these important things to the baby via the umbilical cord. And then the placenta removes unwanted waste, such as carbon dioxide, back via the umbilical cord into the mama-to-be’s bloodstream.

The placenta also does another important job by preventing harmful things from reaching your baby. It works hard to keep viruses and bacteria out of the womb, and stops the baby’s cells transferring back into the mother’s blood.

What about if there are multiple babies?

Do multiple babies mean multiple placentas? Or do the babies share one placenta? Well, the answer is yes and yes…

Depending on the type of twins you’re carrying, it is possible for the babies to share a placenta (if they’re identical and the placenta formation had happened before the embryo split into two or more babies). Otherwise, if the babies are non-identical (also known as fraternal), then each baby will have their own placenta. This is something that can be seen on ultrasounds, so your healthcare provider will discuss it with you if you ask.

It is possible to have complications with your placenta, like having a low-lying placenta. But don’t worry, this is one of the reasons that placenta health is monitored during the course of your pregnancy, and oftentimes, it’ll move to the right position by itself. If your healthcare provider sees something, they’ll help you plan the best course of action.

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