When Does The Placenta Form? All About This Unique Organ

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Parenthood

What You Need to Know About the Placenta

Medically reviewed by Carolyn Kay, M.D. — Written by Sara Lindberg on August 31, 2020

What it is
When it forms
When it takes over
Morning sickness
Complications
Delivery
Takeaway

The placenta grows and changes shape throughout your pregnancy. Any vaginal bleeding, abdominal pain, or back pain may indicate a more serious issue that your doctor needs to address.

Pregnancy involves some pretty wild changes to your body. Did you know that in addition to growing a human, you’re also growing a whole new organ?

While you’re over here dealing with fatigue, cravings, sore boobs, and nausea, behind the scenes, your body is tirelessly working on two very important things: a baby and a placenta.

It’s true: Even early on, the placenta — an organ that’s made of both fetal and maternal tissue — is growing right alongside your little one.

What is the placenta and what is its role?

Your body goes through rapid changes during pregnancy — especially in the early months. To support this process, you need a lifeline to help your baby eat and breathe. The placenta, an organ that develops inside your uterus, provides a connection between you and your baby.

This connection (via the umbilical cord) is what helps to sustain your developing baby throughout pregnancy. Necessary nutrients, oxygen, and antibodies pass from the placenta through the umbilical cord and to your baby.

Your placenta also works in reverse, meaning that it removes waste products and carbon dioxide from the baby’s blood back to your blood.

Since the placenta keeps your blood separate from your baby’s blood, it also helps to keep some bacteria and viruses out of the womb, protecting the fetus from infections.

In addition to being a lifeline to your baby, the placenta also produces and secretes hormones directly into the bloodstream to help support pregnancy and fetal growth. These hormones include:

progesterone
estrogen
oxytocin
human chorionic gonadotropin
human placental lactogen

When does it start and finish developing?

Pregnancy involves a complex series of events, one of which is the formation of the placenta. In general, once the fertilized egg implants in the uterine wall, the placenta begins forming. But the ball starts rolling several days before implantation.

When you ovulate, an egg leaves the ovary to travel through the fallopian tube in hopes of being fertilized. If successful, the ovulated egg meets up with a spermatozoid to begin formation of the fetus. This ovulated egg is called a zygote. Over the course of a few days, the zygote will complete many cell divisions in the fallopian tube.

When the zygote reaches the uterus, these cell divisions continue, and then the zygote becomes a blastocyst. At this stage, some cells begin to form into the placenta and others begin to form the fetus. The blastocyst embeds in the endometrium (aka implantation).

To help support pregnancy, the placenta produces a hormone called human chorionic gonadotropin (hCG). (This is the hormone measured by a pregnancy test, which is why you won’t get a positive test until after implantation.)

Over the course of your pregnancy, the placenta grows from a few cells into an organ that will eventually weigh about 1 pound. By week 12, the placenta is formed and ready to take over nourishment for the baby. However, it continues to grow throughout your pregnancy. It’s considered mature by 34 weeks.

Under normal conditions, the placenta will attach to the wall of your uterus. As pregnancy progresses, it will move with your growing uterus.

When does it take over?

Since the placenta plays such a significant role during pregnancy, it’s helpful to know when it takes over key functions such as hormone production and providing nutrients to your baby.

Although each pregnancy is different, you can expect the placenta to take over around weeks 8 to 12 of pregnancy, with 10 weeks being the average time for most women.

This doesn’t mean that your own hormone production and nutrition aren’t important. In fact, as far as nutrition goes, the placenta “taking over” means that what you consume can now reach your baby via the umbilical cord and placenta.

Those who get pregnant via in vitro fertilization or have hormone deficiencies may need progesterone supplementation in the first trimester. These individuals are often able to stop taking this hormone once the placenta begins its own production.

The placenta and morning sickness

Many women look forward to the second trimester — energy levels rise, hormone levels begin to balance out, and morning sickness subsides. But why does everything seem to get better?

As already mentioned, towards the end of the first trimester (around week 10), the placenta takes over the production of progesterone. This helps reduce nausea and vomiting. For many mamas, this means an end to morning sickness.

And what if your morning sickness doesn’t go way? The placenta may play a role in that case, too.

If you happen to have a higher concentration of a protein called GDF15 in your blood, those frequent bouts of nausea and vomiting may continue for some time.

One study found that women who reported vomiting in the second trimester had a higher level of maternal circulating GDF15 concentrations around week 15 of pregnancy.

The good news? Researchers speculate that high levels of GDF15 could mean that the placenta is developing properly. It could also help explain why some women have hyperemesis gravidarum, which is an extreme form of morning sickness.

Possible complications with the placenta

The placenta usually attaches to the side or at the top of the uterus, but this is not always the case. Sometimes, it can develop in the wrong location or attaches too deeply, leading to placenta disorders.

Even if you experience a healthy pregnancy, complications with the placenta can still occur. That said, these conditions typically happen as a result of factors, like:

genetics
maternal age
high blood pressure
carrying multiples
previous cesarean deliveries
substance use
previous placental problems
abdominal trauma

The good news is your doctor will monitor many of these conditions throughout your pregnancy and work with you on a delivery plan.

Placenta previa is a condition that causes the placenta to grow in the lowest part of the womb, typically within 2 centimeters from the cervical opening. As a result, the placenta covers all or part of the cervix. If the placenta is completely covering the cervix, this can cause bleeding during delivery and most likely will require a C-section for delivery.

Placental abruption is a serious condition that causes the placenta to detach from the wall of the uterus during pregnancy or birth. Symptoms include vaginal bleeding and back or abdominal pain.

Placenta accreta happens when part or all of the placenta is embedded too deep into the uterine wall.

Anterior placenta indicates that the placenta has attached to the front of the stomach. Typically, this isn’t an issue — it’s really just a normal variant — but tell your doctor if you feel decreased fetal movement.

Retained placenta indicates that part of the placenta is still in the womb after childbirth. If the placenta does not deliver within a certain amount of time, generally 30 minutes (but can be as long as 60 minutes), your doctor will perform a procedure to remove the placenta.

Placental insufficiency is diagnosed when the placenta cannot deliver an adequate supply of oxygen and nutrients to the fetus, which can result in pregnancy complications. Your doctor will monitor this condition throughout pregnancy.

Delivering the placenta

If this is your first pregnancy, then you might be surprised to learn that delivering your baby is not the only stage of childbirth. After setting your sights on that beautiful bundle of joy, get ready for stage three of childbirth — delivering the placenta.

During this last leg of delivery, your uterus will continue to contract, allowing the placenta to separate from the uterine wall and eventually through the birth canal for delivery.

If you had a vaginal birth, this process takes about 30 minutes. For mamas that deliver via cesarean, your doctor will remove the placenta.

To move things along, your doctor may ask you to push, or they may massage your uterus. In some cases, they may give you Pitocin to help make the uterus contract. Or, they may recommend a little skin-to-skin contact with your baby to help your uterus contract.

The takeaway

The placenta is an essential organ that provides oxygen and nutrients to your developing baby. It also plays a significant role in the health of your pregnancy.

Throughout your pregnancy, the placenta grows and changes shape. If you experience any symptoms such as bleeding from the vagina, abdominal pain, or back pain, contact your doctor immediately. These may be signs of a more serious condition like placenta previa.

Parenthood
Pregnancy
Pregnancy Health

How we reviewed this article:

SourcesHistoryHealthline has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical journals and associations. We only use quality, credible sources to ensure content accuracy and integrity. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Bleeding during pregnancy. (2019).https://www.acog.org/patient-resources/faqs/pregnancy/bleeding-during-pregnancy
Hammer C. (n.d.). The placenta.http://www.ottawahospital.on.ca/wp-content/uploads/2018/01/the-placenta-for-the-public-web.pdf
How your fetus grows during pregnancy. (2020).https://www.acog.org/patient-resources/faqs/pregnancy/how-your-fetus-grows-during-pregnancy
Perlman N. (2019). Retained placenta after vaginal delivery: Risk factors and management. DOI: http://doi.org/10.2147/IJWH.S218933
Petry C. (2018). Associations of vomiting and antiemetic use in pregnancy with levels of circulating GDF15 early in the second trimester: A nested case-control study. DOI: http://doi.org/10.12688/wellcomeopenres.14818.1
Placental insufficiency. (2020).https://www.mountsinai.on.ca/care/placenta-clinic/complications/placentalinsufficiency
Urner F. (2014). Manual removal of the placenta after vaginal delivery: An unsolved problem in obstetrics. DOI: http://doi.org/10.1155/2014/274651

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Medically reviewed by Carolyn Kay, M.D. — Written by Sara Lindberg on August 31, 2020

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