I found this on Lostpedia.. Thought it was a good catch and we may want to look farther into it...

Jin was very angry at his wife during the events of "...In Translation" (day 33) and the days afterward, and it was only when he left on the raft in "Exodus, Part 1" on day 44 that he forgave her. Therefore, it is unlikely that conception occurred later than day 33.
Ultrasound measurement tests are not particularly reliable. Juliet said "about 8 weeks", which is 56 days, not "53 days" as she said. This could easily be off by a week or even two, allowing the conception to occur before day 33.

I know when I had my ultrasound done the time they said I got pregnant was off by about a week if I remember right and my due date was off by about a week also..

__________________
Please don't give up, Des. Because all we really need to survive is one person who truly loves us. And you have her. I will wait for
you. Always. I love you, Pen

DESMOND: I love you, Penny. I've always loved you. I'm so sorry. I love you!
PENNY: I love you too.
DESMOND: I don't know where I am, but--
PENNY: I'll find you, Des--
DESMOND: --I promise--
PENNY: --no matter what--
DESMOND: --I'll come back to you--
PENNY: --I won't give up--
BOTH: I promise. I love you.