At 13 weeks pregnant, your baby is about the size of a plum. From head to bottom (called crown-rump length), the fetus measures roughly 7.4 centimeters, or just under 2.5 inches, and weighs around 23 grams (just under an ounce). That’s small enough to fit in the palm of your hand, but remarkably detailed for something so tiny.
How Size Is Measured at 13 Weeks
At this stage, fetal length is measured from the top of the head to the bottom of the torso rather than head to toe, because the legs are curled up tightly. This crown-rump measurement is what your provider uses during an ultrasound to confirm gestational age and track growth. By the second half of pregnancy, measurements will shift to head-to-heel length, which is why you’ll see a big jump in reported size later on.
Keep in mind that these numbers represent averages. Your baby may measure slightly larger or smaller and still be perfectly on track. The NIH maintains growth standards based on a large-scale fetal growth study, and providers compare your ultrasound measurements against percentile ranges rather than expecting one exact number.
What’s Developing This Week
Week 13 is packed with structural changes. Bones are beginning to harden, particularly in the skull and the long bones of the arms and legs. The skeleton has been soft cartilage up to this point, so this is the beginning of a gradual process that continues well after birth. The skin is still thin and nearly transparent, but it will start thickening soon.
Vocal cords are forming this week. The baby’s head, which has been disproportionately large compared to the rest of the body, is starting to grow more proportionally. All the major organs, limbs, bones, and muscles are already in place from the previous weeks. The circulatory, digestive, and urinary systems are functioning: the liver is producing bile, and the fetus is already swallowing and releasing small amounts of amniotic fluid.
Over the next week or two, the skin will begin to thicken, fine hair will start growing, and fingerprints will begin forming. The baby will also start bringing its fingers to its mouth and turning its head.
Movement You Can’t Feel Yet
Even though your baby is actively moving at 13 weeks, you almost certainly won’t feel it yet. On an ultrasound, though, the picture is surprisingly lively. You can see wriggling arms and legs, the curve of the spine, and the flickering of a tiny heartbeat. Some parents are caught off guard by how much detail is visible at this stage, including the outline of hands and feet.
Most people don’t feel fetal movement until somewhere between 16 and 22 weeks, and first-time parents tend to notice it later. The baby is simply too small right now for its kicks and stretches to register through the uterine wall and abdominal muscles.
Why Week 13 Is a Milestone
Week 13 marks the very end of the first trimester, the period when fertilization and all major organ development takes place. By now, every organ system has formed and the work shifts from building structures to growing and refining them. This is a meaningful turning point because the risk of miscarriage drops significantly after the first trimester. Most pregnancy losses happen before week 12, so reaching week 13 is a reassuring threshold for many parents.
The placenta is also fully established by this point, handling the critical job of delivering nutrients and oxygen to the fetus. For many pregnant people, this transition coincides with a noticeable improvement in first-trimester symptoms like nausea and fatigue, though the timing varies.
What You Might See on an Ultrasound
If you have a scan around 13 weeks, you’ll typically see the baby’s profile, the rounded head, and the curved line of the spine. The heartbeat shows up as a rapid flicker on screen. Arms and legs are visible and often moving. The baby may be stretching, curling, or changing position during the scan itself.
This is also the window for a screening called the nuchal translucency scan, which measures a small fluid-filled space at the back of the baby’s neck. It’s usually done between weeks 11 and 14 and is part of early screening for certain chromosomal conditions. The scan is quick, noninvasive, and often the first detailed look parents get at their baby’s anatomy.
How Fast Growth Picks Up From Here
At 13 weeks, your baby weighs less than an ounce. Over the next four weeks, that weight will roughly triple. By 20 weeks, the fetus will be around 10 inches long from head to heel. The second trimester is the period of fastest proportional growth, and the size comparisons shift quickly from plum to avocado to banana to mango.
This rapid growth is why nutrition and hydration become increasingly important in the second trimester, even as appetite and energy levels often improve from the early weeks. The building blocks you provide now support bone hardening, brain development, and the formation of billions of new cells each day.