At 8 weeks of pregnancy, the placenta is roughly 10 millimeters thick, about the width of your pinky fingernail. It appears on ultrasound as a thickened area along the uterine wall, still far from the dinner-plate-sized organ it will become at delivery. At this stage, the placenta is just getting started, but it’s already taking on critical work.
Placenta Size Compared to the Embryo
Ultrasound measurements at 8 weeks show placental thickness around 10.2 mm. The embryo itself measures between 16 and 22 mm from head to rump during week 8, so the placenta’s thickness is roughly half the length of the embryo. In terms of overall shape, the placenta at this point looks like a small, flat disc hugging one section of the uterine wall rather than a distinct, rounded organ. It attaches most often to the top, side, front, or back of the uterus.
On a transvaginal ultrasound, you’ll see the embryo and yolk sac clearly, with the placenta visible as a bright, thickened patch of tissue where the pregnancy implanted. Your sonographer can measure its thickness, but it’s too early for the detailed placental checks that happen later in pregnancy. If the placenta appears low at 8 weeks, that’s common and usually resolves on its own as the uterus expands.
What the Placenta Is Doing at 8 Weeks
Even at this small size, the placenta is in the middle of a major transition. For the first several weeks of pregnancy, a structure in the ovary called the corpus luteum produces the progesterone needed to sustain the pregnancy. Around week 7, the placenta begins taking over that job. By 8 weeks, roughly 91% of pregnancies show progesterone levels indicating the placenta has assumed primary hormone production. By week 10, the handoff is complete in virtually all pregnancies.
This hormonal shift is one reason the first trimester can feel so turbulent. The placenta is ramping up its own hormone output while the ovary dials down, and that transition can temporarily affect how you feel.
How the Placenta Grows and Exchanges Nutrients
At 8 weeks, the placenta is building the internal structures it needs to feed the baby for the rest of pregnancy. Tiny finger-like projections called villi are developing rapidly inside the placenta. Some of these villi anchor the placenta firmly to the uterine wall. Others branch out and are surrounded by maternal blood, creating the exchange zone where oxygen and nutrients pass from your bloodstream to the embryo’s.
At this early stage, most of the villi are still immature. Over the coming weeks they’ll branch extensively, dramatically increasing the surface area available for nutrient and oxygen exchange. This branching is what allows the placenta to keep pace with the baby’s growing demands. Think of it like a tree: at 8 weeks, the trunk and main limbs are forming, but the dense canopy of smaller branches won’t fill in until later.
How Much the Placenta Will Grow
To put the 8-week size in perspective, the placenta at full term typically measures about 22 centimeters (roughly 9 inches) in diameter and 2 to 2.5 centimeters thick, weighing around 500 grams (just over a pound). At 8 weeks, it weighs only a few grams. That means the placenta will increase its mass by more than a hundredfold between now and delivery.
Growth happens quickly through the second trimester, with the placenta expanding in both diameter and thickness to match the baby’s increasing need for blood flow and nutrients. A general rule of thumb used by sonographers is that placental thickness in millimeters roughly tracks with gestational age in weeks during the second and third trimesters, so a placenta at 20 weeks would measure close to 20 mm thick. At 8 weeks, the 10 mm measurement fits loosely within that pattern, though early placental measurements are less standardized.
What You Might See on Your 8-Week Scan
Most people have their first ultrasound somewhere between 7 and 9 weeks, so the placenta you see at this visit will look modest. The sonographer is primarily checking for a heartbeat, confirming gestational age, and ruling out ectopic pregnancy. They’ll note where the placenta is attached, but detailed placental evaluation (checking for problems like placenta previa or abnormal thickness) happens at the anatomy scan around 18 to 20 weeks.
If your report mentions the placenta’s location or thickness at 8 weeks, that’s normal documentation. A low-lying placenta at this stage is not a cause for concern on its own. As the uterus stretches over the next several months, the placenta’s position relative to the cervix changes in the vast majority of cases.