A Free easy to use calculator For an avid mother and a busy obstetrician to

know the important dates and duration of your pregnancy (POG plus EDD)

Time in weeks and days of your pregnancy.From only one date entry we

also calculate your expected date of delivery (EDD) which is also

calculated by an obstetrician in your first ante-natal visit. This Obs

calculator is very usefull to obstettrician who don't want to calculate

Expected Date of Dilevery (EDD) and Period of Gestation (POG) manually

and wants or needs a simple tool which only requires one entry. It uses

Parikh's formula for Asian countries accurate to Standard Deviation

13days) and Naegli's formulae for the western countries EDD (Standard

devaition 13days). Primigravida's (First time mothers) are within Plus

minus 7-9days of EDD. it is helpful to consider gestation time as a

range of dates rather than as a single "due date". For instance a

standard deviation of 13 days means that 90% of babies will be born

within two weeks of their EDD and 21% will be born within three days

of it. Only 4% of births will occur on the EDD.
Refrences:
(1). Bergsjø P Denman DW 3rd Hoffman HJ Meirik O. (1990). "Duration

of human singleton pregnancy. A population-based study.". Acta Obstet

Gynecol Scand: 197–207.
(2). Gray H (1962). "Duration of pregnancy.". Stanford Medical

Bulletin 20: 24–8. PMID 13901307.
(3).Dr H. Kieler O. Axelsson S. Nilsson U. Waldenströ (1995). "The

length of human pregnancy as calculated by ultrasonographic measurement

of the fetal biparietal diameter". Ultrasound in Obstetrics &

Gynecology 6 (5): 353–357. doi:10.1046/j.1469-0705.1995.06050353.x.

PMID 8590208.