Week 13

OK here's what's happening in there. Your baby measures about 2.9 inches from crown to rump and weighs around 0.8 ounces. Peach-sized. But the size doesn't tell the real story.

Fingerprints are forming on those tiny fingertips right now. Completely unique to your baby. No one else in human history will share the same pattern. The vocal cords are developing too. They won't make a sound until birth (the larynx needs air to vibrate), but the structures are being built.

That oversized head? Finally getting more proportional to the body. Bones are hardening from soft cartilage into actual bone, especially in the skull and the long bones of the arms and legs. An ultrasound this week would show the skeleton clearly.

Your baby's intestines just finished a weird but necessary phase. During the first trimester, they temporarily extended into the umbilical cord because there wasn't enough room in the abdomen. Now they've migrated back to where they belong. The kidneys are producing urine. The liver is secreting bile. Both organs are rehearsing for life on the outside.

And here's a fun detail: your baby can turn their head now. The neck muscles are strengthening, and the eyes have shifted from the sides of the face to the front. Still fused shut, but unmistakably human-looking.

They can also move their arms freely and may be getting a thumb to their mouth. You can't feel any of it yet. But there's a lot going on in there.

If the first trimester was survival mode, this is where recovery begins. Morning sickness is fading for most people. Food sounds appealing again. That bone-deep exhaustion? Lifting.

Not everyone feels this shift right at 13 weeks. Give it another week or two if you're still dragging. The trend is pointing up.

Your body needs about 340 additional calories per day during the second trimester, according to ACOG. That's roughly a banana with peanut butter and a glass of milk. Think nutrient-dense, not double portions. Your body is building a whole human and needs quality fuel to do it.

Your bump is probably becoming visible. The uterus is rising above the pubic bone, and your provider can feel it during abdominal exams now. You might notice a dark vertical line appearing down the center of your belly. That's the linea nigra, and it shows up in more than 90% of pregnancies. It's caused by increased melanin production from pregnancy hormones. Totally normal. Fades after birth.

Round ligament pain can start around now. Sharp, sudden twinges when you stand up or roll over in bed? The ligaments supporting your growing uterus are stretching under increasing weight. Changing positions slowly helps. A warm bath helps too. If the pain doesn't go away with rest or comes with bleeding, call your provider.

This is a good time to add gentle exercise if you haven't already. Walking, swimming, prenatal yoga. A 2024 dose-response meta-analysis in BMC Public Health found that physical activity during pregnancy was associated with reduced gestational diabetes risk, with stronger benefits at higher activity levels. Talk to your provider about what makes sense for you.