Expert: Catherine Beier MS CBE - 6/3/2011

Question
QUESTION: I have a question would i be able to tell who fathered my child if i had sex with somebody before my period and then one after i got off my period and that month i missed my period? i got an ultrasound but i was wondering how could it date me off my lmp if i didnt member it i also heard that if your ultrasound says 13wks and 5days that really means you conceived 11wks and 5days ago but is that the same method if you dont member your lmp...

ANSWER: Hello Michelle,

You've asked several questions, so let's address them one at a time:

Pregnancy length is typically calculated as 40 weeks from the first day of your last period, meaning that you are actually only pregnant for 38 weeks, as conception typically occurs about two weeks before your next period is to begin.  This is based on having a regular 28 day cycle where you ovulate on day 14, so for women with long and/or irregular cycles, then traditional pregnancy dating methods from LMP are not usually accurate.

The most accurate was to date a pregnancy is by early transvaginal ultrasound between 8-10 weeks. It has a standard error of measure of +/- 1 day.  This is because every fetus develops at much the same rate in the first 12 weeks of pregnancy, cells divide and multiply at the same rate. Once the placenta is fully formed and takes over supporting the pregnancy, this is when factors such as genetics, nutrition, prenatal care, etc begin to differentiate how quickly some babies grow and is why some are born at 5 pounds and others at 11 pounds despite two mothers being pregnant for the same length of time.

So, even if you do not know the date of your last period, an early ultrasound (again, best between 8-10 weeks, decreasing significantly in reliability after 12 weeks) can still give accurate information on how far along you are.


Yes, if an ultrasound says 13 weeks, 5 days, this means conception most likely occurred 14 days before, making the baby's fetal age 11 weeks, 5 days.  This is the same whether or not you know your LMP.  This is because the average woman's luteal phase, the length of time between ovulation and the start of her next period, is 14 days.  Anything between 12-16 days is considered average.


Now, regarding knowing paternity if you had intercourse with two men - one before your period and one after it - yes, it should be possible but would depend upon at what point in your cycle you had intercourse with both of them.  Real dates would be needed to determine paternity with any level of certainty.  If the first was after you had already ovulated, and the second was before your next ovulation, then yes, it should be apparent which one is the father, but not more information as to the dates in question.



Best,
Catherine Beier MS CBE
http://www.GivingBirthNaturally.com

Yes, if an ultrasound says 13wks, 5days, that means

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

QUESTION: I dont know when i ovulated i know i had sex with guy 1 a couple days before my period in october then after i got off my period then i started havin sex with guy #2 in november and thats the month i missed my period..... but if i conceived before my period in october wouldnt a ultrasound date me futher?

Answer
Michelle,

Yes, if you conceived in October, then it would date you further along than you are.  Since you're not providing any sort of date, just speaking in terms of months, it's impossible to be more specific without more detailed information.


Best,
Catherine Beier MS CBE
http://www.GivingBirthNaturally.com