Ultrasound dating a bit off?

[anovulation]

I had an ultrasound done yesterday for dating purposes since I have no idea when my last cycle was. I know for sure the dates I had sex though. The ultrasound put me at 7w5d with a due date of April 13. By my calculations that take into consideration the days we had sex, my due date should be between April 3 and April 6. I just find it unlikely

Comments

[willsjoy]

I believe sperm can live up to a week, but regardless, the later date is better anyway. Less chance of being pressured into an induction before the baby is ready. My early u/s had me 10 days later than I thought and it was very accurate.

[mela_chan]

Agreeing with willsjoy that the slightly later date is good, since it will help avoid early induction pressure and talk.

Sperm has been found to be able to live in the uterus/fallopian tubes for 7 days (maybe as long as 10), depending on the conditions within your body. Early ultrasounds can also be off by 3-5 days, according to my OB.

[lilyofthewest]

Those dates could reasonably give you a conception date anywhere between July 10th and July 20th, possibly (though incredibly unlikely) one as late as July 25. That would give a due date range anywhere between April 1st and April 11th (maybe as late as the 16th on a way outside bet).

Dating ultrasounds have a +/- 8% error rate, which at 7w5d is plus or minus 4.32 days -- so really dating at anywhere between 7w0d and 8w3d -- which calculates out to a due date range of April 9th to April 19th.

Once you figure that a due date is a wild guess anyhow, since baby will probably pick whatever day suits it between 37 and 42 weeks.

[mothermelete]

It could also be late implantation, or any other number of things.
Seriously, by the day I had sex vs the due date, I knew it was off, but I was grateful for the later due date. I also thought I had sex far too early in my cycle to conceive, and either my cycle was abnormally short or that sperm stayed alive in there for a good week.

[so_there]

if the sperm lived for 5 days that puts conception at the 20th so this is really only 2 days off, right? I can't imagine 2 days makes much of a difference, regardless of how early the u/s is.