At 13 weeks, your baby measures about 7 to 8 centimeters (roughly 3 inches) from head to rump, making it about the size of a kiwi fruit. This is a crown-to-rump measurement, meaning it doesn’t include the legs, which are still tucked up against the body. Your baby weighs roughly 23 to 25 grams, or just under an ounce.
Crown-Rump Length Through Week 13
Fetal size at this stage is measured from the top of the head to the bottom of the torso because the legs are too flexed to measure reliably. Based on international growth standards, the median crown-rump length changes noticeably even within a single week:
- 13 weeks, 0 days: 6.8 cm (2.7 inches)
- 13 weeks, 3 days: 7.3 cm (2.9 inches)
- 13 weeks, 6 days: 7.9 cm (3.1 inches)
That’s roughly a full centimeter of growth in just seven days. This rapid pace is typical of the late first trimester, when the baby is doubling in size every few weeks. If your ultrasound measurement falls a bit above or below these numbers, that’s normal. These are 50th-percentile values, meaning half of all babies measure larger and half measure smaller at the same gestational age.
What Your Baby Looks Like at 13 Weeks
By now the baby’s body proportions are shifting. Earlier in pregnancy the head made up nearly half the total length, but at 13 weeks it starts to grow more proportionate to the rest of the body. Facial features are increasingly defined: the eyes have moved closer together toward the front of the face, and the ears are nearly in their final position on the sides of the head.
The vocal cords are forming this week. They won’t produce sound until after birth, but the tissue is already laying the groundwork. Tiny fingerprint ridges are beginning to develop on the fingertips, and all 20 baby teeth have formed as tooth buds beneath the gums. The arms and legs have lengthened enough that the baby can now bring its hands together in front of its body.
Movement You Can’t Feel Yet
On ultrasound, a 13-week fetus is surprisingly active. Complex whole-body movements, including combinations of arm and leg extensions and flexions, are visible as early as 8 or 9 weeks, and by 13 weeks these movements are frequent and varied. You might see the baby stretching, flexing its limbs, or shifting position during a scan.
You won’t feel any of this yet. At under an ounce, the baby is too small and the amniotic fluid cushions every motion. Most first-time mothers don’t notice fetal movement until 18 to 22 weeks. If you’ve been pregnant before, you may recognize it a few weeks sooner, closer to 16 weeks.
Organ Development at This Stage
The kidneys are functional and beginning to produce small amounts of urine, which gets released into the amniotic fluid. The baby also swallows amniotic fluid, practicing the swallowing reflex it will need after birth. This cycle of swallowing and urinating helps maintain the right volume of amniotic fluid. By around 20 weeks, the baby’s urine will make up the majority of that fluid.
The intestines, which earlier in development temporarily extended into the umbilical cord because there wasn’t room in the abdomen, have now migrated back into the body. The liver has started producing bile, and the spleen is beginning to make red blood cells.
The Placenta Takes Over
One of the most significant shifts at 13 weeks is happening not with the baby but with the placenta. Up until the end of the first trimester, a temporary structure called the corpus luteum (formed on the ovary after ovulation) handles most of the hormone production that sustains the pregnancy. Around week 12 or 13, the placenta is mature enough to take over that role entirely.
This handoff is the reason many people start feeling noticeably better right around now. The nausea, fatigue, and food aversions that dominate the first trimester often ease once the placenta assumes hormone production. It’s not instant for everyone, but the transition is well underway at 13 weeks.
Changes in Your Body
Your uterus is now about the size of a grapefruit and is beginning to rise above the pelvic bone. In earlier weeks it was entirely tucked within the pelvis, which is why your belly may not have been showing. At 13 weeks, you or your provider may be able to feel the top of the uterus (the fundus) just above the pubic bone during an exam. For some people, this is when a small bump first becomes visible, though timing varies widely depending on body type and whether this is a first pregnancy.
You may also notice a dark vertical line forming on your abdomen, running from the navel downward. This is caused by the same hormonal shifts driving melanin production, which can also darken the areolas and cause patches of darker skin on the face. These pigment changes typically fade after delivery.
Where 13 Weeks Falls in Pregnancy
Week 13 is the final week of the first trimester. The American College of Obstetricians and Gynecologists defines the first trimester as lasting through 13 weeks and 6 days, with the second trimester officially beginning at 14 weeks and 0 days. This is the stage when all major organs have formed and the focus shifts from organ development to growth and maturation. The risk of miscarriage drops significantly after the first trimester, which is one reason many people choose this point to share pregnancy news.