The Placenta: Your Baby's First Lifeline
- Jul 4th, 2026
Midwife Talks
When people think about pregnancy, they often picture the growing baby. But behind every healthy pregnancy is an extraordinary organ that rarely gets the attention it deserves—the placenta. The placenta is one of the most remarkable organs the human body ever creates. It develops specifically for pregnancy, supports your baby's growth every second of every day, and is delivered shortly after your baby is born. In many ways, it functions as your baby's lungs, kidneys, digestive system, liver, and immune system all at once. Let's take a closer look at this incredible organ and why it plays such an essential role throughout pregnancy. The placenta begins forming just days after conception. Once the fertilized egg reaches the uterus, it implants into the uterine lining. The outer layer of cells, called the trophoblast, begins invading the lining of the uterus and develops into the placenta, while the inner cells become the embryo. Over the first trimester, tiny finger-like projections called chorionic villi grow deep into the uterine lining. These villi become surrounded by the mother's blood, creating an efficient exchange system where oxygen and nutrients can pass to the baby while waste products are carried away. By approximately 12 weeks of pregnancy, the placenta has taken over hormone production from the corpus luteum (in the ovary) and continues to grow alongside your baby until around 34-36 weeks, when it reaches its mature size. At full term, the placenta typically weighs about one pound (450-600 grams) and measures roughly 8-10 inches across. The placenta has so many vital functions that it performs simultaneously to support the growing fetus. It delivers oxygen and nutrients since your baby doesn't breathe or eat inside the womb. Oxygen from your lungs and nutrients from the food you eat cross the placenta and travel through the umbilical cord to nourish your growing baby. It removes waste products like carbon dioxide and metabolic byproducts. These travel back through the placenta into your bloodstream, where your body eliminates them. It produces pregnancy hormones, basically acting as a powerful endocrine organ by producing hormones that support pregnancy, including: It protects the baby by acting as a selective filter, blocking many bacteria and harmful substances. As amazing as it is, though, it is not an impenetrable barrier. Certain viruses, medications, alcohol, nicotine, and some environmental toxins can still cross into the baby's circulation. The placenta also transfers maternal antibodies, especially during the third trimester, helping provide your baby with immune protection during the first few months after birth. During your anatomy ultrasound, one of the things your provider carefully evaluates is where the placenta is attached. Many placentas implant on the front (anterior), back (posterior), top (fundal), or side (lateral) of the uterus. All of these are usually completely normal. However, it becomes an issue when the placenta develops too close to the cervix or covers the cervix. When it's "too close" (less than 2 cm away from the internal cervical opening) it creates a condition called a low lying placenta. If it covers the cervix, it's a condition known as placenta previa. Fortunately, a low lying placenta diagnosed early in pregnancy often "moves" upward as the uterus expands. In reality, the placenta doesn't physically migrate; rather, the lower uterine segment stretches as pregnancy progresses, increasing the distance between the placental edge and the cervix. This is why many women diagnosed with a low lying placenta at 20 weeks have a completely normal placental position by the third trimester. The next step would be to have a follow up ultrasound around 32-34 weeks of pregnancy to see where the placenta is located at that time and make the appropriate clinical decision. Some providers may recommend a 28 week ultrasound, but if the placenta is still low lying an additional ultrasound will be necessary. If the placenta still covers or lies very close to the cervix near term, a cesarean birth is usually the safest option for both mother and baby. As labor begins, the cervix must open to allow the baby to pass through the birth canal. If the placenta blocks this opening, several serious complications can occur: One of the placenta's many jobs is producing hormones that help ensure your baby receives enough nutrients. Some of these hormones—including human placental lactogen, placental growth hormone, progesterone, cortisol, and estrogen—naturally make the mother's cells less responsive to insulin. This process is called insulin resistance. For most pregnancies, the pancreas simply produces more insulin to compensate. However, if the pancreas cannot keep up with the increased demand, blood sugar levels begin to rise, resulting in gestational diabetes. In other words, gestational diabetes isn't caused by eating too much sugar—it develops because the placenta's hormones create a normal physiologic state that some bodies cannot fully compensate for. This is also why gestational diabetes usually resolves after delivery, once the placenta is born and those hormone levels rapidly decline. Because the placenta is responsible for producing the hormones that increase insulin resistance, the timing of gestational diabetes screening is no coincidence. In early pregnancy, insulin resistance is relatively mild. As pregnancy progresses and the placenta enlarges, insulin resistance steadily increases, becoming most significant during the late second and third trimesters. This is why routine screening for gestational diabetes is typically performed between 24 and 28 weeks of pregnancy. By this point, placental hormone production has increased enough that gestational diabetes will usually become apparent if the pancreas cannot produce sufficient insulin to overcome the normal pregnancy-related insulin resistance. Some individuals are screened earlier in pregnancy if they have significant risk factors, such as a history of gestational diabetes, obesity, polycystic ovary syndrome (PCOS), prediabetes, or a strong family history of diabetes. It is important to remember that an early normal test does not eliminate the need for repeat screening at 24-28 weeks, because insulin resistance continues to rise as the placenta grows. The placenta also plays a central role in the development of preeclampsia, a pregnancy complication characterized by high blood pressure and signs of organ dysfunction. One of the most fascinating discoveries in obstetrics over the past few decades is that preeclampsia is fundamentally a condition caused by the placenta. Although the symptoms appear in the mother—such as high blood pressure, headaches, vision changes, swelling, or abnormal lab work—the condition actually begins much earlier, during the placenta's development. In a healthy pregnancy, specialized placental cells called trophoblasts invade the wall of the uterus during the first trimester. Their job is to remodel the mother's spiral arteries, transforming these small, high-resistance blood vessels into large, low-resistance vessels capable of delivering a continuous supply of oxygen-rich blood to the growing placenta and baby. In pregnancies that later develop preeclampsia, this remodeling process is often incomplete. The spiral arteries remain narrower than they should be, reducing blood flow to the placenta. As the pregnancy progresses and the baby's oxygen and nutrient demands increase, the placenta experiences periods of relative stress and low oxygen (placental ischemia). Rather than simply being affected by this reduced blood flow, the placenta actively responds by releasing proteins into the mother's bloodstream. These proteins interfere with normal blood vessel function by reducing the activity of natural growth factors, including vascular endothelial growth factor (VEGF) and placental growth factor (PlGF), which normally help keep blood vessels healthy and relaxed. As these placental proteins circulate throughout the mother's body, they cause widespread endothelial dysfunction—meaning the delicate inner lining of blood vessels no longer functions normally. This leads to blood vessels becoming narrower and less able to relax, causing blood pressure to rise. It also explains why preeclampsia affects so many organs, including the kidneys, liver, brain, lungs, and placenta itself. This understanding also explains one of the most important facts about preeclampsia: the only definitive cure is delivery of the placenta. While medications can lower blood pressure and magnesium sulfate helps prevent seizures, they do not stop the underlying disease process. Once the placenta is delivered, the source of these harmful proteins is removed, and the mother's body gradually begins to recover. Although blood pressure often improves within days, some women continue to require close monitoring for several weeks postpartum as their cardiovascular system returns to its pre-pregnancy state. Researchers continue to study why some placentas develop abnormally while others do not. We now know that preeclampsia is a complex condition involving placental development, maternal cardiovascular health, immune regulation, genetics, and environmental factors. In most cases, it is not something a mother caused or could have prevented. Early prenatal care and regular blood pressure monitoring remain the best ways to detect the condition before it becomes severe. One of my favorite parts of birth is examining the placenta after delivery. A lot of midwives will take the time to examine it with the families and teach them about it. It's fascinating for families to see the fetal side and the maternal side—how the umbilical cord is connected to it, and how Wharton's jelly is protective of the blood vessels inside it. We call it a "placenta tour." Seeing the placenta brings a new appreciation for the incredible work it has quietly performed over the past nine months. The placenta provides valuable information about how the pregnancy functioned. Its size, shape, blood vessels, membranes, and umbilical cord can offer important clues about fetal growth, maternal health, and pregnancy complications.
How the Placenta Develops
What Does the Placenta Actually Do?
Why Placental Location Matters
The Placenta and Gestational Diabetes
The Placenta's Role in High Blood Pressure and Preeclampsia
Every Placenta Tells a Story
References