A full-term placenta is roughly the size of a dinner plate. It typically measures about 22 centimeters (nearly 9 inches) in diameter, is 2.5 centimeters (1 inch) thick at the center, and weighs around 470 grams, or just over one pound. It’s round to oval in shape, with a fetal-facing surface area of about 286 square centimeters.
How the Placenta Grows During Pregnancy
The placenta doesn’t start out dinner-plate sized. It grows steadily alongside the baby, and ultrasound measurements of its thickness offer a reliable window into that growth. At 11 weeks, the placenta averages about 13.5 millimeters thick. By the second trimester, that rises to roughly 25 millimeters. In the third trimester it reaches about 35 millimeters, and by 40 weeks it measures around 39 millimeters at its thickest point.
A useful rule of thumb: placental thickness in millimeters roughly tracks the gestational age in weeks. So at 20 weeks, you’d expect it to be close to 20 millimeters thick, and at 30 weeks, close to 30 millimeters. Your provider may note the thickness during routine ultrasounds, but it’s rarely the focus unless something looks unusual.
What Counts as Too Thick
An abnormally thick placenta, called placentomegaly, is generally defined as one thicker than 40 millimeters (4 centimeters) on ultrasound. Some researchers use slightly different cutoffs: at least 4 centimeters in the second trimester or 6 centimeters in the third. Another common approach is flagging any placenta above the 90th or 95th percentile for gestational age. One large study placed that 95th-percentile line at about 36 millimeters at 32 weeks and 40 millimeters at 36 weeks.
A thick placenta isn’t a diagnosis on its own. It’s a finding that can prompt your provider to look more closely. Possible causes include infections, fetal anemia, chromosomal differences, and maternal conditions like diabetes. In many cases, no underlying cause is found.
How Gestational Diabetes Affects Placenta Size
Gestational diabetes tends to make the placenta heavier. In one study comparing pregnancies with and without gestational diabetes, the average placental weight was about 592 grams in the diabetes group versus 562 grams in the normal group, a difference of roughly 30 grams. Interestingly, the babies in both groups weighed about the same, meaning the placenta was disproportionately large relative to the baby.
This pattern, a higher placental weight relative to birth weight, shows up even in milder forms of glucose intolerance during pregnancy. The placenta in diabetic pregnancies also tends to show more immature blood vessel structures and increased blood vessel formation. These changes reflect the extra metabolic work the placenta does when maternal blood sugar runs high.
Placenta Size in Twin Pregnancies
You might assume twins would produce a placenta twice the size of a singleton pregnancy, but that’s not quite how it works. The combined placental weight in twin pregnancies is consistently less than double the singleton weight at the same gestational age. Twin placentas grow fastest between 24 and 36 weeks, then hit a plateau around 37 weeks. Singleton placentas, by contrast, gain weight more evenly throughout pregnancy.
Twins may share a single placenta (monochorionic) or each have their own (dichorionic), and this distinction matters more for health outcomes than total placental weight. When twins share a placenta, uneven blood flow between them is a risk that providers monitor closely with ultrasound.
Why Placenta Size Matters
The placenta is the baby’s sole supply line for oxygen and nutrients, so its size has a direct relationship to how well it can do that job. A placenta that’s too small may struggle to keep up with the baby’s demands, particularly in the third trimester when growth accelerates. This can contribute to lower birth weight or growth restriction.
An unusually large placenta can signal that the organ is compensating for something, whether that’s higher blood sugar, lower oxygen levels, or an infection it’s working harder to manage. In both directions, size alone doesn’t determine outcomes. The placenta’s internal structure, blood flow patterns, and how well it’s attached to the uterine wall all play a role.
After delivery, your provider will examine the placenta to check that it was delivered intact. They’ll look at its overall size, shape, the attachment point of the umbilical cord, and whether the membranes are complete. If anything looks unusual, or if there were complications during pregnancy, the placenta may be sent to a lab for a more detailed examination.