Expert: Catherine Beier MS CBE - 5/31/2009

Question
QUESTION: I was having irregular periods so do not no my lmp date but I do no the date I had intercourse which was the saturday the 27th and sunday the 28th september 2008. I had a dating scan which showed my little boyo to be due on saturday the 18th july 2009 so was is the saturday that i concieved. this question is so important to be answer'd im desprite to know please help many thanks louise

ANSWER: Louise,

Based on your due date, neither of those dates would be a possible conception date.  You most likely conceived around Oct. 25.

Best,
Catherine

---------- FOLLOW-UP ----------

QUESTION: So is it not true that a sperm can live for upto 7 days inside the female body then it can take upto 12 days for the fertalized egg to implant itself then you are classed as being pregnant and the 40 week count down begins?? I am very confused thank you very much for your last answer would really apprieate 1 more response many thanks

Answer
Louise,

No, you are definitely confused on how pregnancy length is calculated.  First, sperm can only live in the reproductive tract for up to 5 days, which is the maximum.  3 days or less is average.


The 40 week pregnancy countdown typically begins from the date of your last period.  However, if you have irregular periods and/or a long cycle, this method isn't accurate at determining a due date.  Then, an early ultrasound is the best method to date the pregnancy and is accurate within +/- 3 days.


When an ultrasound is used to calculate your due date, you would count back 38 weeks from that day to find your probable conception date.


The pregnancy countdown begins from the date of your last period, before you are even pregnant, because ovulation & conception typically don't occur until about 14 days before your next period is due to start. This method of calculation doesn't work well for most women because it's only accurate if you have a 28 day cycle and ovulate on day 14, which most women don't.  But, a due date is just an estimate in any case.


For example, if someone's LMP was 12-1-08 and she had a 28 day cycle, she would most likely conceive around 12-15-08 and would be due 40 weeks from her LMP, which would be 9-7-09.


As another example, if someone's LMP was 12-1-08 and she had a 40 day cycle, she would most likely conceive around 12-17-08 (about 14 days before her next period was to start) and would be due around 9-19-08.


So, in your case, you would count back 38 weeks from your due date to find your conception date, which would be 10-25-08.



Best,
Catherine