At 13 weeks pregnant, your baby measures about 6.7 centimeters (roughly 2.6 inches) from crown to rump, which is about the size of a lemon or a peapod. That measurement covers the top of the head to the bottom of the torso, since the legs are still tucked up and difficult to measure consistently. Your baby weighs roughly 23 to 25 grams at this point, or just under an ounce.
What 13 Weeks Looks Like on Ultrasound
If you have an ultrasound around this time, you’ll see a baby that actually looks like a baby. The head is still large relative to the body, making up about a third of total length, but the rest of the body is catching up fast. Arms, legs, fingers, and toes are all visible and fully formed. You may even see your baby moving on screen, stretching or flexing those new limbs, though those movements are far too small for you to feel yet.
Limb movements at this stage are becoming more coordinated rather than the earlier random twitches, but it will typically be several more weeks before they’re strong enough for you to notice from the outside.
Key Developments Happening This Week
Size is only part of the story. At 13 weeks, your baby’s internal systems are hitting some important milestones. The kidneys have begun producing urine, which gets released into the amniotic fluid. This is the start of a cycle that continues throughout pregnancy: your baby swallows amniotic fluid and the kidneys filter it back out, helping to maintain the fluid levels in the womb.
Ridges are forming in the palms and on the soles of the feet. These ridges will eventually become your baby’s unique fingerprints and footprints, patterns that are set during pregnancy and never change. Bones that were previously soft cartilage are beginning to harden, particularly in the skull and the long bones of the arms and legs. The intestines, which earlier developed partly inside the umbilical cord because there wasn’t enough room in the abdomen, have now moved fully into the body cavity.
The Placenta Takes Over
One of the biggest behind-the-scenes changes at 13 weeks involves the placenta. For the first 12 weeks, a structure called the corpus luteum (formed in your ovary after ovulation) handled most of the hormone production needed to sustain the pregnancy. By the end of the first trimester, the placenta has fully taken over that job, producing the hormones that keep the pregnancy going and managing the exchange of oxygen, nutrients, and waste between you and your baby.
This hormonal handoff is one reason many people start feeling better around this time. The surge of early pregnancy hormones that triggered nausea and exhaustion begins to stabilize once the placenta is running the show.
Where You Are in Pregnancy
Week 13 marks the official start of the second trimester. You’re a third of the way through your pregnancy, and for many people, this is when the fog of the first trimester starts to lift. Morning sickness often begins to ease, though some people still deal with it for a few more weeks. Energy levels typically improve compared to the deep fatigue of the first trimester.
Your uterus is growing and beginning to rise out of the pelvis. You may notice your lower belly feeling firmer, and some people start to show around this time, especially in a second or later pregnancy. Others won’t have a visible bump for several more weeks.
Screening Tests Around This Window
If you haven’t already had a nuchal translucency (NT) scan, week 13 is near the end of the window for this screening. The test is performed between 11 and 13 weeks, when the baby measures between 45 and 84 millimeters from crown to rump. It uses ultrasound to measure a small pocket of fluid at the back of the baby’s neck, which can help assess the risk of certain chromosomal conditions. After 14 weeks, the fluid gets reabsorbed and the measurement is no longer reliable, so this is a time-sensitive screening.
How Growth Changes From Here
Your baby has been growing rapidly, but the second trimester is when things really accelerate. At 13 weeks, you’re looking at a baby that weighs under an ounce. By the end of the second trimester at 28 weeks, that same baby will weigh around two pounds. The organs that have spent the first trimester forming will spend the next several months maturing and becoming functional. For now, though, the major structural work is largely done. Your baby has all the essential organs, all the limbs, and a face that’s becoming more distinctly human each day.