At 13 weeks, your baby is about the size of a plum, measuring roughly 2.5 inches from crown to rump and weighing just under an ounce. This is the final week of the first trimester, and your baby has shifted from a curled, tadpole-like shape into something that looks unmistakably human, with a defined profile, visible fingers and toes, and a head that’s starting to become more proportional to the body.
Size and Proportions
Crown-to-rump length at 13 weeks averages about 7.4 centimeters (just over 2 inches), with a weight around 23 grams (0.81 ounces). Those measurements can vary slightly depending on the source, but the plum comparison holds up well for visualizing your baby’s size in your hand.
The head still accounts for roughly a third of the baby’s total length, but the body is catching up fast. The limbs have lengthened considerably over the past few weeks. Fingers and toes are fully separated, and the arms are now long enough that the hands can reach the face. If you catch the right moment on an ultrasound, you might see your baby with a hand near their mouth.
Facial Features Taking Shape
By 13 weeks, your baby’s face looks remarkably like a tiny human face. The eyes, which started on the sides of the head, have migrated closer together toward the front. The eyelids are fused shut and will stay that way for several more weeks. The ears have moved into their final position on the sides of the head, and the nose and lips are clearly formed. A profile view on ultrasound at this stage often shows a recognizable forehead, nose bridge, and chin.
Skin and Surface Details
The skin is still extremely thin and translucent. Blood vessels are visible beneath the surface, giving the baby a reddish appearance. This transparency will gradually change as the skin thickens over the coming weeks. Fine details are already forming beneath the surface, including the early patterns that will become unique fingerprints. A soft, downy hair called lanugo will begin covering the body soon, helping to regulate temperature and protect the skin.
Organ Development at 13 Weeks
Most major organs are formed by this point, and the focus shifts from building structures to refining how they work. The kidneys became functional around week 12 and are now producing small amounts of urine, which gets released into the amniotic fluid. This is completely normal and actually contributes to the fluid surrounding the baby.
The intestines, which earlier in development temporarily bulged into the umbilical cord because there wasn’t enough room in the abdomen, have migrated back into the body cavity and are settling into position. The liver has begun producing bile, and the pancreas is starting to make insulin. The vocal cords are also forming this week, though your baby won’t use them until birth.
The placenta is now fully established as the baby’s life-support system, delivering oxygen and nutrients while filtering waste. It took the entire first trimester for the placenta to develop, and from this point forward it handles the heavy lifting of keeping your baby nourished.
Movement You Can’t Feel Yet
Your baby is surprisingly active at 13 weeks, stretching, turning, and flexing newly formed muscles. The movements are still too small for you to feel (most women don’t notice kicks until 16 to 22 weeks), but on ultrasound you might catch arm waves, leg kicks, or even hiccup-like motions. The sucking reflex won’t mature until much later, around 32 to 36 weeks, but the groundwork for coordinated mouth movements is already being laid. Some babies at this stage can be seen opening and closing their mouths.
Can You Tell the Sex at 13 Weeks?
This is one of the most common questions at this stage, and the answer is: sometimes. All babies develop a small structure between the legs called a genital tubercle, often referred to as a “nub,” which appears between weeks 11 and 13. On ultrasound, the angle of the nub relative to the spine can indicate sex. A nub angled more than 30 degrees from the spine suggests male, while one that lies parallel or nearly parallel suggests female.
Accuracy improves rapidly in this window. At 11 weeks, ultrasound-based sex identification is correct about 70% of the time. By 12 weeks that jumps to nearly 99%, and by 13 weeks accuracy reaches essentially 100% in studies, though real-world results depend on the baby’s position and the quality of the image. If you’re curious, a blood-based screening test (often done between weeks 10 and 13) can also reveal sex with high accuracy.
What You Might Notice in Your Own Body
At 13 weeks, your uterus has grown to the point where it’s rising just above the pubic bone. For many women, this is when a small bump first becomes visible, especially in tighter clothing. Some women, particularly first-time mothers, won’t show for a few more weeks. The nausea and fatigue that defined the first trimester often begin to ease around this time, though that timeline varies widely from person to person.
On ultrasound at this stage, the baby is large enough to see clearly but small enough to fit in a single frame. You can typically make out the head, spine, limbs, and sometimes the profile of the face. If your provider measures the baby during a scan, you’ll likely see them draw a line from the top of the head to the bottom of the spine, which is the crown-to-rump length used to confirm gestational age.