TL;DR
- Your baby is about 13 cm — the size of a pear.
- Baby is developing a layer of body fat for warmth and energy.
- You may experience back pain and trouble finding a comfortable sleeping position.
- Start sleeping on your side — a pillow between your knees can help.
Your Baby This Week
At 13 cm, your baby is now the size of a pear and starting to develop something important: body fat. This layer of fat, called adipose tissue, will eventually help regulate your baby's body temperature and serve as an energy reserve. Right now it's just the very beginning — your baby is still lean and translucent — but this fat will continue to accumulate throughout the rest of pregnancy.
The umbilical cord is growing thicker and stronger, delivering nutrients and oxygen while removing waste. It's a remarkable lifeline — about 50 cm long by now and containing two arteries and one vein, all protected by a thick, jelly-like substance called Wharton's jelly.
Your baby's cartilage skeleton is continuing to harden into bone, and sweat glands are beginning to develop. The fingerprints — those completely unique patterns — are forming on tiny fingertips this week. No two sets will ever be the same, not even in identical twins.
Your baby is also practicing breathing movements, inhaling and exhaling amniotic fluid to help the lungs develop. These "practice breaths" are essential preparation for life outside the womb, even though actual breathing won't happen until birth.
Your Body This Week
As your belly grows, you may notice that your center of gravity is shifting. This can lead to back pain, especially in the lower back, as your muscles work harder to support the extra weight out front. Good posture, supportive shoes, and gentle stretching can all help.
Sleep might be getting more complicated. If you've always been a back or stomach sleeper, adjusting to side sleeping can feel awkward. But as your uterus grows, sleeping on your side — particularly the left side — improves blood flow to the placenta. A pillow between your knees, behind your back, or a full body pillow can make side sleeping much more comfortable.
You might notice increased vaginal discharge, which is normal. Your body produces more discharge during pregnancy to help protect the birth canal from infection. As long as it's clear or white and doesn't smell unusual, there's nothing to worry about.
Some people experience vivid, bizarre dreams during pregnancy. Hormonal changes, disrupted sleep, and the emotional weight of impending parenthood can all contribute to a more active dream life. If you're having strange dreams, you're in good company.
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Common Questions at Week 17
Is it safe to sleep on my back?
At 17 weeks, occasional back sleeping is unlikely to cause problems. However, as your uterus gets heavier, sleeping on your back can put pressure on a major blood vessel called the vena cava. Most providers recommend transitioning to side sleeping during the second trimester. If you wake up on your back, just roll over — no harm done.
Why do I keep getting headaches?
Headaches are common in the second trimester due to hormonal shifts, increased blood volume, dehydration, and changes in sleep patterns. Stay hydrated, eat regularly, and rest when you can. Acetaminophen (Tylenol) is generally considered safe during pregnancy, but check with your provider.
Should I be concerned about stretch marks?
Stretch marks are largely determined by genetics and skin type. They're extremely common and usually appear on the belly, breasts, hips, and thighs as skin stretches rapidly. Keeping skin moisturized can help with itching, but no cream can guarantee prevention. Many stretch marks fade significantly after birth.
This Week's Tip
Start sleeping on your side if you haven't already — a pillow between your knees can help tremendously. This position takes pressure off your back and improves circulation to your baby. If you tend to roll onto your back during the night, try placing a pillow behind you as a gentle barrier. Some people find a full body pillow to be a game-changer for second trimester sleep comfort. It might take a few nights to adjust, but your body and baby will thank you.
Sources
- American College of Obstetricians and Gynecologists (ACOG). How Your Fetus Grows During Pregnancy.
- Mayo Clinic. Fetal Development: The 2nd Trimester.
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