This is the week most women discover they are pregnant - whether it be a missed period, the earliest of pregnancy symptoms, or a positive pregnancy test. For me, it was coming home with Petrie's third picture.
Here is what's going on inside: At 4 weeks pregnant, the blastocyst (or ball of cells that will become your baby) is working furiously to implant itself in the uterine lining. This will ensure a safe home for your little one over the next nine months. The amniotic cavity and placenta are also forming during pregnancy week four. Fetal development by and large is still in the earliest stages at this point.
Your baby is incredibly miniscule, about .014 to .04 inches long (somewhere between the size of a poppy seed and an organge seed). Over the next few weeks, your baby will slowly start resembling a small tadpole with eyes, rather than an egg. This early stage of pregnancy is often noted as the beginning of the embryonic period.