I discovered an interesting fact today that I had missed in my 34 years: When a woman says she is 8-weeks pregnant, that is the time since her last period. It is not the time since conception.

Two weeks ago, I signed up for an online service that sends you updates on your child’s development in the womb. Two weeks ago, our embryo was the size of a poppy seed; last week he or she was the size of a sesame seed. (I’m looking at bagels differently these days.) This week, our baby is the size of a lentil and I literally found that out while eating leftover lentil soup. In the future I’m not going to open those emails while eating lunch, at least until the baby grows to be too big to fit in my lunch bag.

But I realized there was a problem when I entered our due date (we don’t have an official due date yet, but I counted nine months from fertilization). The website kept telling me that we were two weeks further along than I thought. At first I manipulated the due date so that the development chart showed the right number of weeks, but then the due date was way too early.

So, I Googled it. “How do you determine how far along your pregnancy is?” I asked. Every response detailed the fact disclosed above. Of course, that’s not relevant to our situation. B. has been on ovulation suppression drugs for months.

I should have started with the IVF sites because I finally found that a good rule of thumb to convert an IVF pregnancy’s development (in terms of weeks pregnant) to the scale normally used is to add two weeks to the date of conception.

So we’re six weeks into this thing and not four, like I thought. That means that the ultrasound B. is having tomorrow will determine the viability of the pregnancy; we don’t have to wait another two weeks. That makes me both more excited and more nervous.