When is my Due Date?

When you tell people that you are pregnant, the first question that they will probably have is ‘When are you due?’ However, many women are unsure as to how to work out their due date as it can be very confusing. You may have heard people referring to a pregnancy as being nine months but also 40 weeks: when does the pregnancy actually start?

The length of an average pregnancy is calculated as being 40 weeks long. However, the pregnancy actually begins about 2 weeks before conception as the due date of your baby is calculated from the first day of your last menstrual period (LMP) (The day you first began to bleed). It may sound unbelievable that a pregnancy can begin before the sperm has actually reached the egg but this is the way that all medical staff will measure the length of your pregnancy. The main theory behind this is that you will never be 100% sure of the date you conceived, even if you knew when you were ovulating, you could not be 100% certain of when your partner’s sperm actually fertilised the egg as sperm can survive for three days inside your body.

If you know the date of your LMP, you can use the chart below to determine your due date.  Locate the date of your LMP (in blue) and the date below is your Estimated Due date (EDD) (in red).

January 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Oct/Nov 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7
February 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28  
Nov/Dec 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5  
March 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Dec/Jan 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5
April 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Jan/Feb 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4
May 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Feb/Mar 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 1 2 3 4 5 6 7
June 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Mar/Apr 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6
July 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Apr/May 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 7
August 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
May/Jun 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7
September 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30  
Jun/Jul 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 7
October 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Jul/Aug 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7
November 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Aug/Sep 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6
December 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Sep/Oct 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 7

However, this method of calculating your due date presumes that your menstrual cycle is 28 days long and regular. Many women have longer, shorter or irregular periods and this will affect the result of your due date.

Although a pregnancy is calculated as 40 weeks in length, a baby is considered to be full term (fully grown and ready to be born) between 38 and 42 weeks. Therefore it is important to remember that your baby’s estimated due date (EDD) can still vary. You may be tempted to count down the days but  the chance of your baby arriving on the due date is slim.

At your first booking in appointment with your midwife, your EDD will be calculated and written in your hand-held antenatal notes. In the UK, all pregnant women are offered a dating scan between 10 and 14 weeks.  During this scan the sonographer (person carrying out the scan) will check how your baby is progressing and growing, will be able to tell you how many babies you are carrying and also from looking and measuring your baby inside the womb, will be able to give you a more accurate EDD.

If you do not know when the date of your LMP, your midwife will arrange an ultrasound scan as early as possible to determine your baby’s due date.

by Jenny, mum to William and James

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