At 13 weeks pregnant, your baby is about the size of a plum, measuring roughly 2.5 inches long from crown to rump and weighing around 2.5 ounces. This week marks the start of the second trimester, and your baby is going through a remarkable stretch of development.
Exact Measurements at 13 Weeks
Fetal size is measured from the top of the head to the bottom of the buttocks, called crown-rump length. At the start of week 13, the average measurement is about 68 millimeters (just under 2.7 inches), and by the end of the week it reaches close to 79 millimeters (just over 3 inches). That’s a full centimeter of growth in just seven days, which gives you a sense of how rapidly things are moving.
Weight at this stage is approximately 2.5 ounces, or about 70 grams. For context, that’s roughly the weight of a small egg. From here, weight gain accelerates significantly as your baby starts building fat and muscle throughout the second trimester.
What’s Developing This Week
Size is only part of the story. At 13 weeks, your baby’s vocal cords are forming, and the head, which has been disproportionately large compared to the body, is starting to become more proportional. Tooth buds have already been in place since around week 9, and by next week, fingerprints will begin to form on those tiny fingertips.
Your baby’s kidneys are now producing small amounts of urine, which gets released into the amniotic fluid. At the same time, your baby is swallowing that fluid, a process that helps the digestive and respiratory systems develop. After about 20 weeks, fetal urine will actually make up most of the amniotic fluid. It sounds strange, but this constant cycle of swallowing and urinating is essential practice for life outside the womb.
Movement You Can’t Feel Yet
Your baby started moving around week 12, stretching, kicking, and shifting position inside the uterus. These movements help joints, muscles, and bones develop properly. But at 13 weeks, your baby is still too small for you to feel any of it.
Most first-time mothers don’t notice movement until around 20 weeks. If you’ve been pregnant before, you may pick up on those fluttery sensations, called quickening, as early as 16 weeks. The movements are happening right now though, and they’d be visible on an ultrasound.
The Placenta Takes Over
One of the biggest behind-the-scenes shifts happens right around this time. Until the end of the first trimester, a temporary structure called the corpus luteum handled most of the hormone production needed to sustain your pregnancy. By week 13, the placenta has fully taken over that job, delivering nutrients and oxygen to your baby and producing the hormones that keep everything on track.
This handoff is a big reason many people start feeling better in the second trimester. The nausea and deep fatigue that defined the first 12 weeks often ease up once the placenta assumes control. Not everyone gets that relief immediately, but for many, the next few weeks bring noticeably more energy.
What You’d See on an Ultrasound
If you have an ultrasound at 13 weeks, you’ll see a baby that actually looks like a baby. The limbs are visible and moving, the head is rounded, and you can often make out the profile of the face. The body is still translucent, but all the major structures are in place.
One common question is whether you can find out the sex at 13 weeks. Research from the Fetal Medicine Foundation found that ultrasound-based sex determination reached 100% accuracy at 13 weeks when performed by experienced specialists using specific techniques. At 12 weeks, accuracy was about 99%, and at 11 weeks it dropped to around 70%. The method involves measuring the angle of a small structure called the genital tubercle, which points upward in males and stays flat or angled downward in females. That said, standard anatomy scans are typically scheduled around 18 to 20 weeks, so most parents learn the sex then. External genitalia don’t show a clear size difference between male and female until after 14 weeks, which is one reason many providers prefer to wait.
How 13 Weeks Compares to Nearby Weeks
Growth during this stretch of pregnancy is fast. At 10 weeks, your baby was about the size of a strawberry. By 16 weeks, they’ll be the size of an avocado, roughly 4.5 inches long. The jump from plum-sized at 13 weeks to avocado-sized just three weeks later shows how quickly length and weight accumulate once the second trimester begins.
The organs that formed during the first trimester are now maturing and starting to function. Over the coming weeks, your baby will develop the ability to hear sounds, facial expressions will become more complex, and bone tissue will continue to harden. The foundation is built. The second trimester is about growth, refinement, and practice.