At 13 weeks pregnant, your baby measures about 2.5 inches (6.8 cm) from crown to rump and weighs roughly 2.5 ounces. That’s about the size of a plum. This week marks the very end of the first trimester, and your baby has crossed a significant threshold: every major organ system is in place, and the focus shifts from building new structures to growing and refining the ones already there.
Crown-to-Rump Length at 13 Weeks
Fetal size is measured from the top of the head to the bottom of the tailbone, called crown-rump length (CRL). At exactly 13 weeks and 0 days, the 50th percentile CRL is 67.8 mm, or just under 2.7 inches. There’s a healthy range, though. Babies at the 5th percentile measure around 59.2 mm, while those at the 95th percentile come in at about 76.5 mm. That’s a spread of nearly two-thirds of an inch from the smallest to the largest, all considered normal.
Your baby’s legs aren’t included in this measurement because they’re still curled up tight against the body. If you could stretch them out, total length would be closer to 3 inches. By the end of the second trimester, ultrasounds will switch to measuring individual body parts (head, abdomen, thighbone) instead of crown-to-rump length, since the baby becomes too curled and large to measure in one straight line.
What Your Baby Looks Like Now
At 13 weeks, your baby has shifted from looking like a generic embryo to looking unmistakably human. The head is still disproportionately large, making up about a third of the total body length, but the body is starting to catch up. Facial features are becoming more defined: the eyes have moved from the sides of the head toward the front, the ears are nearly in their final position, and a nose, upper lip, and lower lip are all distinct. Tiny fingerprints are forming on fingertips that now have individual digits instead of webbed paddles.
The skeleton is transitioning from soft cartilage to harder bone, starting with the skull and long bones of the arms and legs. Your baby can make fists, bend at the elbows, and even suck a thumb, though these movements are reflexive rather than intentional. If you could peek inside, you’d see thin, translucent skin with visible blood vessels underneath.
Organ Development at 13 Weeks
The kidneys are a good example of how rapidly things progress at this stage. Tiny filtering units in the kidneys (glomeruli) developed back around weeks 8 to 9, and by week 13 the bladder is visible on ultrasound. Full tubular function, where the kidneys actively process fluid, kicks in after week 14. Once that happens, your baby will begin producing small amounts of urine that get excreted into the amniotic fluid, which your baby then swallows and recycles. This loop is essential for maintaining a healthy volume of amniotic fluid throughout pregnancy.
The intestines, which earlier bulged into the umbilical cord because there wasn’t enough room in the abdomen, have now moved back inside the body and are beginning to practice the wave-like contractions they’ll need for digestion after birth. The liver is producing bile. The pancreas is starting to make insulin. The vocal cords are forming in the larynx, though your baby won’t use them until that first cry.
What a 13-Week Ultrasound Can Show
If you have an ultrasound around 13 weeks, your sonographer can see a surprising amount of detail. The four chambers of the heart are visible, along with the heart valves and the major arteries leaving the heart. The brain shows clear structures: a visible midline, early brain regions, and the fluid-filled spaces (ventricles) that cushion the developing brain. The stomach appears as a small dark bubble on screen, confirming your baby is swallowing amniotic fluid.
Facial bones are identifiable too. The nasal bone, upper jaw, lower jaw, and eye sockets with their tiny lenses can all be measured and assessed. The diaphragm is visible on both sides, and the umbilical cord insertion point shows two arteries carrying blood to the placenta. Many of these structures are checked specifically because abnormalities at this stage can flag chromosomal conditions or structural problems early.
One common measurement taken between weeks 11 and 14 is nuchal translucency, the fluid-filled space at the back of the baby’s neck. A thicker-than-expected measurement can indicate increased risk for Down syndrome and certain heart defects, though it’s a screening tool rather than a diagnosis.
The Placenta Is Fully in Charge
By 13 weeks, the placenta has been running the hormonal show for several weeks already. Early in pregnancy, a temporary structure called the corpus luteum (a cyst on the ovary where the egg was released) produces the progesterone needed to sustain the pregnancy. The handoff to the placenta, sometimes called the luteal-placental shift, happens around 7 weeks. By that point, the placenta is producing enough progesterone on its own that the corpus luteum is no longer essential.
This transition is one reason many women notice their worst nausea and fatigue starting to improve around weeks 12 to 14. The hormonal turbulence of the first trimester begins to stabilize as the placenta settles into a steady production rhythm. The placenta at 13 weeks is also handling oxygen and nutrient delivery, waste removal, and immune protection, functioning as your baby’s lungs, digestive system, and kidneys all at once until those organs are ready to work independently.
How 13 Weeks Compares to Surrounding Weeks
Growth at this stage is rapid. Just one week earlier, at 12 weeks, your baby was closer to 2 inches long. One week later, at 14 weeks, it’ll be about 3.4 inches. That means your baby is growing roughly half an inch per week right now, a pace that will continue through the early second trimester before gradually slowing in the third trimester, when weight gain becomes the priority over length.
Weight gain follows an even steeper curve. At 13 weeks your baby weighs about 2.5 ounces. By 20 weeks, that number jumps to roughly 10 ounces. By 30 weeks, it’s around 3 pounds. Most of the fat that gives newborns their round appearance isn’t laid down until the final 8 weeks of pregnancy, so right now your baby is lean and long relative to their eventual birth size.