Expert: Shan Yi Sheng - 2/19/2009

Question
QUESTION: Hi again,

I have a question regarding measuring a pregnancy. When someone says they
are 5 weeks pregnant or 8 weeks pregnant, are they talking about 8 Weeks
LMP or from conception onwards?

Secondly, What is LMP? I know it means Last menstrual period but does that
mean the first day of your period or the last day of your period?

Thirdly, when one web searches for ultrasound pictures of pregnancies and
you type 5 weeks or 7 weeks, these pictures which appear, are they dated as
per LMP?  Won't ultrasound pictures differ because sometimes it takes time
for the egg to attach to the uterus? as in instead of one week it might take
two weeks for conception and therefore a typical 8 week pregnancy might
show a 7 week old pregnancy instead?

ANSWER: Hi Christina again,

LMP means reckoned from the first day of the last period.
Fertilization takes place ± 10-14 days later.
So if the LMP is april 1st, the pregnancy starts at april 10th -14th.
Or: if you LMP was january 1st than your pregnancy - after LMP is now
about 7-8 weeks. At the ultrasound you see an embryo of 5-6 weeks.
Not to difficult ...if you know.
Right ?
bests,
dr  Shan

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

QUESTION: Hi Dr. Shan

So you are saying that if you include LMP the pregnancy is 7-8 weeks, but the ultrasound pictures one sees is of 5-6 weeks. When a gynaec shows you the ultrasound pictures they date it as per conception, not including LMP, Right? So if an ultrasound photo says 6 weeks below, it's excluding LMP, its date as per conception. Have I understood it correctly?

Secondly, Fertilization or ovulation time you mention is 10 - 14 days, some websites state 16 days, is this fine?

Thank you

Answer
Hi Christina  again,

this is not standardized. Manny ObGyn doc use LPM weeks,  others use weeks of conception. There is a two week difference.
What the ultrasound shows can be called in conception weeks OR in LMP weeks, depending on the doc, not on the size.
Why is this ? It's because most women know exactly the first day of there LMP and have to guess about the day of conception (although sometimes they are very sure about that also).
I think that using a double standard is one of the many unfavourable riddles of medicin.

hope this helps you a bit.
Up to the next question !
dr Shan