Answer: At 13 weeks your baby weighs about 23g (0.8oz), and is about 7.4cm (2.9in) long from crown to rump, which is around the length of a pea pod. All of her organs and muscles are in place, though they still have plenty of developing to do.
This month, your baby's facial muscles are getting a workout as her tiny features form one expression after another. She'll be squinting and grimacing, and she may even have hiccups now and then as she practises the movements necessary for breathing. Her body is starting to develop a fine coating of hair, known as lanugo, which will help to keep her warm. Most babies shed this hair before the birth.
Your baby's brain is developing, too, particularly the part that's responsible for memory and problem-solving. By the end of this month, her hearing will have developed to the point that she can hear your heart beating, and may even be able to make out the muffled sound of your voice. She's growing fast, and by week 16, she will have reached about 11.6cm (4.6in) in length, about the size of an avocado.