YOUR BABY AT WEEKS 1 AND 2

No, there’s no baby or even an embryo in sight (at least not yet) — just an anxious egg and a whole bunch of eager sperm at their respective starting gates. But in weeks one and two of pregnancy — the week of and immediately following your last menstrual period — your body is working hard to gear up for the event that paves the way for the baby: the big O — ovulation. Right now your uterus has begun preparing for the arrival of a fertilized egg, though you won't know for sure that egg has successfully matched up with sperm until next month.

Calculating Your Due Date

How can you call this your first week of pregnancy if you're not even pregnant? It’s extremely hard for your practitioner to pinpoint the precise moment pregnancy begins (i.e., when sperm meets egg). While there’s no mistaking the start of your period, the exact day of ovulation can be hard to nail down. What’s more, sperm from your partner can hang out in your body for several days before your egg comes out to greet it. Likewise, your egg can be kept waiting for a day or two for tardy sperm to make their appearance. So in order to give all pregnancies some standard timing, most practitioners use the first day of your last menstrual period as the starting line of your 40-week pregnancy. Still confused? Think of it as a head start — you're clocking in roughly two weeks of pregnancy before you even conceive!

Source: What To Expect