Track your pregnancy with our app: https://wte.onelink.me/85iI/29dt1g98'>https://wte.onelink.me/85iI/29dt1g98 Your baby is developing rapidly, with most key body systems and vital organs fully formed by now. This week, your baby's heartbeat may be audible during your checkup.

Learn more about week 12 of pregnancy: https://www.whattoexpect.com/pregnanc...

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It's a landmark week for your baby, since most of the key body systems and vital organs are fully formed by now. Your little one will spend the rest of your pregnancy bringing those systems up to full speed and maturation so they'll be able to function to perfection once your bundle is delivered into your arms.

And your baby can now brag about this: Doubling in size in just three weeks, crowntorump length is about 2 to 2 ¼ inches, about the size of a lime. A crowning achievement, baby's brain structure is complete and now it's time for massive gray matter growth.

Your baby's thyroid starts to work on its own, the pituitary gland has started producing hormones and the pancreas has begun to synthesize insulin. Not to be outdone, your baby's bone marrow is churning out white blood cells, the critical weapons needed to fight infection once your little warrior is on the outside and no longer protected by your immune system.

Meanwhile, muscles in the digestive tract are getting primed, practicing their contractions so they can process food after birth. Yet while baby's digestive system is rapidly developing, yours is slowing down, the better to absorb nutrients, but resulting in gas, bloating and oh yes, constipation.

The best development of all, if you haven't already heard it: This is the week that your baby's heartbeat will come through loud and clear at your 12week checkup, and that will be music to your ears.