3 weeks pregnant: what is happening this week

Fertilisation, if it happens, happens in this week - and then a journey of several days follows before anything attaches to anything. The fertilised egg travels down the fallopian tube while dividing, arrives in the uterus as a ball of cells, and only later embeds in the lining. That gap between fertilisation and implantation is why a pregnancy test taken now will read negative no matter how accurate it is: the hormone tests look for is produced after implantation, not before it. There is nothing to feel and nothing to see. Some people report symptoms in this week; the honest position is that a cycle produces a lot of sensations and this one is indistinguishable from the others at the time, however clear it looks in hindsight. What is genuinely useful right now is boring: the folic acid dose, and, if there is any chance of a pregnancy, treating alcohol and smoking as though there already is one. Both of those are NHS guidance for the whole first trimester rather than something to start once a test turns positive, precisely because the earliest weeks are the ones you cannot know about while they are happening.