The very beginning — a tiny spark settling in.
About the size of a poppy seed — or a tiny snail, or a grain of sugar. 0.2 cm (0.1 in), under 1 g (under 0.1 oz).
What's happening this week. The fertilized egg has settled into the uterus and the placenta is starting to form. It's just a cluster of cells — but every instruction for eyes, heart and tiny toes is already written.
Implantation is complete. The placenta begins to form. The neural tube is taking shape.
By the end of this week the embryo has three layers — each one becomes a different part of the body.
What your partner can do this week: Talk about who you'll tell, and when. There's no right time — only the one that suits you both. Find out how pregnancy care starts where you live: who to contact first, and how soon. Start one shared list for the things that need doing, so nobody has to be the one who remembers.