If you have regular 28-day menstrual cycles, there are two ways to calculate your due date.

Naegele’s rule involves a simple calculation: Add seven days to the first day of your LMP, then subtract three months.

No-one doubts that you know when your last period was.

pregnancy dating by lmp-30

Pregnancy lasts an average of 280 days (40 weeks) from the first day of your last menstrual period (LMP).

The first day of your LMP is considered day 1 of pregnancy, even though you probably didn't conceive until about two weeks later (fetal development lags two weeks behind your pregnancy dates).

Ovulation will have taken place roughly 2 weeks after this, so in effect pregnancy is about 38 weeks long.

This is just an average, and it is considered normal to deliver anywhere between 37 weeks and 42 weeks from LMP.

When your dates are calculated from the LMP, this presumes that fertilisation occurs 2 weeks later.

In many women, an ultrasound scan may shift their expected date of delivery (EDD), which comes as a surprise.

accurate dating of pregnancy at times becomes difficult especially in the presence of conflicting menstrual data and patient presenting for late ultrasound, this ppt explores the ways to arrive at a correct due week or date in such a scenario do we mature early? A London-based study of over 122 000 pregnancies with spontaneous onset of labour Roshni R Patel1,2, Philip Steer3, Pat Doyle1, Mark P Little4 and Paul Elliott4 IJE vol.33 no.1 107-113 © International Epidemiological Association 2004; all rights reserved. The median gestational age at delivery was 39 weeks in Blacks and Asians and 40 weeks in white Europeans. Menstrual age Overestimates the gestational age Wilcox M et al BMJ 1993;307(6904):588-91 10-45% do not provide correct information 18% with certain dates have significant difference with USG age Geirsson et al, Br J Obstet Gynaecol. Naegeles’ rule does not correlate with actual conception date….

Meconium stained amniotic fluid, which is a sign of fetal maturity, was statistically significantly more frequent in preterm Black and Asian infants and term Black infants compared with white European infants. Baird et al498 normal cycles luteal phase 7-19 days 10% ovulate on D14 Wicox et alonly 30% of normal cycle women are fertile between 10-17 days 50% pregnancy are unintended LMP recall is error prone Indian Husbands more aware of their wives’ LMP!

How long pregnancy lasts varies somewhat between women.

The due date toward which you have been working is based on pregnancy lasting 40 weeks or 280 days from the date of the last menstrual period (LMP).

Sometimes the difference comes when a bleed which was thought to be the last period was in fact some bleeding in early pregnancy and you are one month further on than initially expected.