A. Using the ‘period dates’:

1. Take the FIRST DAY of the Last Menstrual Period [LMP]. 

2. Moving the top disc on the Pregnancy Calculator side of the Obdisk 2-IA, line up the LMP arrow with the date of the patient's LMP. 

3. You can read off the fetal gestational age in weeks, throughout pregnancy, by following the calendar around carefully. For a reading that is as accurate as possible, adjust the discs slightly if necessary to match up the days/weeks exactly. 

4. You can read off the Expected Date of Delivery [EDD] at the triangle marked “EDD DUE DATE”. The date of this simple calculation is therefore based only on the date of the Last Menstrual Period. 

B. Using the dating scan result, when available:

If a 'dating scan' is performed [using ultrasound], simply turn the top disc [on the Pregnancy Calculator side] so that the number of weeks and days agree with the date of the scan. Then read off the fetal gestational age, and the Expected Date of Delivery, by following the calendar around carefully. [Adjust the discs slightly if necessary to match up the days/weeks exactly.]  

Notes:

1. The Pregnancy Calculator side of the Obdisk 2-IA is based on a pregnancy of 280 days from the LMP if there have been regular 4-week cycles, but the EDD would need a suitable adjustment for irregular cycles and a leap year. 

2. The 'period dates' and scan dates may, or may not, agree. Different hospitals may have different policies about how to deal with a discrepancy between these dates, but a doctor or midwife will be able to explain this to the patient if necessary. 

3. You can use the Pregnancy Calculator side of the Obdisk 2-IA to calculate 'period dates' and scan dates - but not both at the same time. [The pocket-sized 3-disc Obdisk 3 model is specifically designed for this purpose, and shows any discrepancy between the two calculations.] 

4. Because of the traditional method of estimating human pregnancy in the UK, a woman is considered to be 6 weeks pregnant [from the date of her Last Menstrual Period] when she has only been pregnant for 4 weeks [from conception]. The due date is generally estimated at 40 weeks [280 days] after the LMP, and 38 weeks [266 days] after conception.

Infant Age in Weeks - Instructions For Use: 

1. Moving the top disc on the Infant Age side of the Obdisk 2-IA, line up the BIRTH DAY arrow with the date of the baby’s birth.

2. You can read off the baby’s age in weeks, throughout his first year of life, by following the calendar around carefully. For a reading that is as accurate as possible, adjust the discs slightly if necessary to match up the days/weeks exactly. 

Notes: 

1. The ‘Infant Age in Weeks’ side of the Obdisk 2-IA is a Perpetual Calendar, which means that it will continue on forever into eternity. Each complete revolution represents one calendar year of 365 days [ie 52 weeks plus one day]. 

2. Leap Year: Suitable adjustment of the discs will be necessary for a leap year, which has 366 days [ie 52 weeks plus two days]. 

3. The ‘Infant Age in Weeks’ side of the Obdisk 2-IA can be used for so much more than calculating the age of an infant! You might want to use it to plan when clinic appointments are due… to calculate how many weeks there are until you have to submit your assignment… or even to count down the weeks until your wedding day!

This is the Price of an Obdisk 2-IA [Pocket-Sized Pregnancy Calculator + Infant Age], including Delivery within the UK...