At 13 weeks pregnant, your uterus is roughly the size of a grapefruit. It has grown from its pre-pregnancy size (about the size of a small pear) to fill most of your pelvic cavity, and the top of the uterus, called the fundus, now sits right at or just above the pubic bone. This is the point where the uterus begins to rise out of the pelvis, and many women notice their lower belly starting to round out visibly for the first time.
Where the Uterus Sits at 13 Weeks
Before pregnancy, your uterus is tucked deep in the pelvis behind the bladder. By 12 to 13 weeks, it has expanded enough that the top edge reaches the symphysis pubis, the spot where the two sides of the pubic bone meet at the front of your pelvis. Over the next few weeks, the fundus will continue rising into the abdominal cavity, which is why most pregnant people start “showing” sometime in the early second trimester.
Because the uterus is still relatively low at 13 weeks, your healthcare provider won’t measure fundal height with a tape measure yet. That measurement typically starts around 20 weeks, when the top of the uterus is high enough in the abdomen for the number in centimeters to roughly match the number of weeks you are pregnant. At 13 weeks, your provider may instead feel for the fundus with their hands during a prenatal exam to confirm that growth is on track.
How Much the Uterus Has Grown
A non-pregnant uterus is about 7 to 8 centimeters long and weighs around 60 grams. By 13 weeks, it has roughly doubled in length and increased significantly in volume to make room for a growing fetus, a placenta, and an increasing amount of amniotic fluid. The uterine walls, which started out thick and muscular, gradually thin as they stretch. In a healthy pregnancy, the wall thickness decreases by about a centimeter between the first and second trimesters as the organ expands outward.
Your baby at this stage has a crown-to-rump length of about 6.8 centimeters (roughly 2.7 inches), or a bit shorter than a AA battery. But the uterus itself is considerably larger than the baby, because it also houses the fluid and placental tissue that support growth. All of this together is what gives the uterus its grapefruit-like dimensions.
What You Might Feel as the Uterus Grows
The rapid expansion at 13 weeks brings some physical sensations that can catch you off guard if you’re not expecting them. The most common is round ligament pain. Two bands of tissue called the round ligaments run from the sides of the uterus down into the groin, and as the uterus stretches these ligaments get longer and wider. The result is a sharp, stabbing, or pulling sensation in the lower pelvis or groin area, often on one side. It typically lasts only seconds to minutes and tends to flare when you change positions quickly, cough, or sneeze.
Round ligament pain is especially common in the second trimester because that’s when the uterus enters its most rapid growth phase. Many women describe it as aches, cramps, or sudden spasms. While uncomfortable, it’s a normal side effect of the uterus doing exactly what it needs to do.
You may also notice increased pressure on your bladder, since the expanding uterus is still sitting low in the pelvis and pressing directly on it. Frequent urination is typical at this stage. As the uterus rises higher over the coming weeks, bladder pressure often eases temporarily before returning in the third trimester when the baby drops lower again.
How Size Varies Between Pregnancies
Not everyone’s uterus will measure identically at 13 weeks. If you’ve been pregnant before, the uterus and abdominal muscles tend to stretch more easily, so you may show earlier and your uterus may feel slightly larger. Your body type, the position of the uterus (tilted forward versus backward), and the amount of amniotic fluid all play a role too.
If you’re carrying twins, the uterus is generally larger at every stage compared to a singleton pregnancy, though the difference at 13 weeks can be subtle since both babies are still quite small. The more noticeable size difference with multiples tends to become obvious in the mid-second trimester as the total volume of babies, fluid, and placental tissue grows faster than in a single-baby pregnancy.
What Comes Next
From here, growth accelerates. By 16 weeks the uterus will be roughly halfway between the pubic bone and the navel, and by 20 weeks the fundus typically reaches the belly button. That predictable rise is what makes fundal height a useful screening tool later in pregnancy. At 13 weeks, you’re at the starting line of visible growth, and most people find that their belly changes noticeably over the next four to six weeks as the uterus moves firmly into the abdominal space.