Hi Lucy,
        Gestation is 266 days +/- 6 days, from conception... the LMP/EDD
method is only of minor value as it has nothing to do with
ovulation/conception, you take the LMP, add 7 days and subtract 3 months,
this is based on the "average" cycle of 28 days, ovulating on day 14. But
few women are average!!
         So for this woman with an LMP of 7/10 add 7 days, 17th and subtract
three months - April 17th due date... but ovulation prediction from that?
How long were her cycles? if 28 days then maybe ovulated on July 24th, but
if longer or shorter??? Hard to say she may have ovulated anytime from 7/10
through ???
        From "The New NO Pill NO Risk Birth Control" by Nona Aguillar, which
is all about the "sympto-thermo method" charting of temperature, mucous, and
cervical position to prevent pregnancy, there is a way to calculate fertile
times, but again this can vary due to illness, stress or other factors. Only
the NEXT period(or birth date) is linked to ovulation *not* the previous one
(true menstruation, as opposed to break through bleeding, occurs 12-16 days
*after* ovulation bc of the hormone chain triggered *by* ovulation)
        So take the average length of the cycles and subtract 20, the next
10 days give you the fertile range. For a 28 day cycle this means fertile
range of day 8, [which is assuming the last bleed was in fact 'menstruation'
and not an annovulatory cycle break through bleed], to day 18 with a
probable ovulation between day 14-18. The range is a few days before
ovulation because sperm can live many days (how many people realize that
conception occurs more often NOT during the sex act but many hours or even
days later when the sperm finally reach the egg?). Noting that you CAN'T get
pregnant AFTER ovulation -- before ovulation yes, sperm can live, but the
EGG DIES within 24 hours of being released.
        To recap: average length minus 20, next 10 days fertile range for
your mom if she's average of 28 days then her fertile range would have been
7/18 - 7/28 with an expected menstruation missed on 8/7. But none of that is
written in stone!
         This mom would be more accurate to wait until the baby is born and
then count back 266 days +/- 6 and see where she's at then, but if she had
sex with multiple partners in that range it's anyone's guess! Of course if
the baby is premature then she's got nothing to go by... there are sooo many
factors that she's NOT going to be able to tell by LMP and EDD with any kind
of accuracy. If she's really got to know an amnio could tell the paternity...
         It's my personal opinion that the LMP/EDD method is why so many
babies in the US are "late"-- they aren't late, the method of estimating due
dates is unreliable. It seems silly, to me, to even give *a* date, they
ought to give a range!
         Why does this matter? Well many doctors tend to get ancy about
lateness and size/ health of baby, but that often means inducing labor with
medications, and monitors and IVs and other interventions that all make for
more likely poor outcomes for the health of mothers and babies, not to
mention breastfeeding.
          Sorry this got so long -- I think it's a shame that women in our
society don't know these basic facts about their own bodies and don't have
the information they need to make decisions about their own lives.
Especially when this is sooooo easy to track!

Good luck to her!

anne in Oregon.