At 13 weeks, a fetus measures roughly 67 to 80 millimeters from crown to rump, which is about 2.6 to 3.1 inches. That’s approximately the size of a lemon. Weight at this stage is typically around 23 to 25 grams, or just under an ounce. This week marks the very beginning of the second trimester, and it’s a point where several major developmental shifts are underway.
How Size Is Measured at 13 Weeks
Fetal size this early in pregnancy is measured as “crown-rump length,” meaning from the top of the head to the bottom of the torso. Legs aren’t included because they’re curled up tightly and difficult to measure consistently on ultrasound. At 13 weeks exactly, the crown-rump length averages about 67 to 68 millimeters, and by the end of the week (13 weeks, 6 days) it reaches 79 to 80 millimeters. That’s roughly a centimeter of growth over just seven days.
If you have an ultrasound scheduled around this time, this measurement is one of the main things your provider will check. It’s used to confirm or adjust your due date, and it’s most accurate for dating a pregnancy during the first trimester.
What’s Developing at This Stage
Week 13 is more than just a size milestone. The fetus is actively building the systems it will need after birth. Vocal cords are forming this week. The bones, which started replacing soft cartilage weeks earlier, are continuing to harden. The digestive and urinary systems are functional enough that the fetus is now swallowing amniotic fluid and producing small amounts of urine, recycling it back into the amniotic space.
The circulatory system is working, and the liver is producing bile. All of this is happening in a body that could still fit comfortably in the palm of your hand.
Movement and Reflexes
At 13 weeks, the fetus is moving around. Early movements tend to be jerky and random, but they’re starting to look more deliberate as the nervous system matures. Some babies even begin sucking their thumb in the womb around this time, which helps develop the sucking reflex they’ll need for feeding after birth.
You won’t feel any of this yet. Most people don’t notice fetal movement until somewhere between 16 and 22 weeks, when the baby is large enough for its kicks and rolls to reach the uterine wall with enough force to register.
The Nuchal Translucency Scan
If you’re offered an ultrasound around 13 weeks, it may be a nuchal translucency scan. This is a screening test that measures the small pocket of fluid at the back of the fetus’s neck. The fluid space is measured in millimeters, and a larger-than-typical measurement can signal an increased chance of certain chromosomal conditions. During the same scan, the provider checks for the presence of the nasal bone and takes the crown-rump length measurement.
This scan is performed between 11 and 13 weeks, when the fetus measures between 45 and 84 millimeters, so 13 weeks is near the end of the window. If your provider hasn’t mentioned it yet, it’s worth asking whether this screening is still available to you.
The Placenta Takes Over
One of the most significant behind-the-scenes changes at 13 weeks has nothing to do with the fetus’s size. By the end of the first trimester, the placenta takes over hormone production from the corpus luteum, the temporary structure on the ovary that sustained the pregnancy in its earliest weeks. This hormonal handoff is a big reason many people notice their nausea and fatigue starting to ease as they enter the second trimester. It doesn’t happen overnight for everyone, but the shift is underway.
Changes You Might Notice in Your Body
At 13 weeks, the top of the uterus is typically just above the pubic bone. That means the pregnancy is starting to rise out of the pelvis, and some people begin to show a visible bump around this time, though this varies widely depending on body type and whether it’s a first pregnancy. You might notice your regular pants feeling tighter even before a bump is obvious to anyone else, because the uterus is expanding and your abdominal organs are shifting slightly to accommodate it.