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When will my baby arrive?

Advice on the accuracy of your estimated due date and how close to this you can expect your baby to arrive.
One of the first things everyone wants to know when they find out they're pregnant is when their baby will be arriving and with good reason, adjusting to the idea of becoming a parent (again or for the first time!), planning things like family visits and maternity leave and working out when you should have the nursery ready and your bag packed are all so much easier if you have a date to work around. However, while your healthcare provider is likely to give you a due date you shouldn't rely on this too much. In fact, what your doctor will work out is known as an 'estimated due date' (EDD) which may give you some idea of its accuracy!

How is my due date worked out?

Your doctor is likely to work out your EDD based on a rule that was developed by a Dr Nagele in the mid 19th century (yes that long ago!). He calculated that the average gestational period of a pregnancy is 266 days (which equates to 38 weeks) from conception, or 280 days (40 weeks) from the first day of your last menstrual period (LMP) and so formed a rule which calculates your baby's expected arrival based on this. You can do this calculation yourself by subtracting 3 months from, and then adding 7 days to, your LMP. For example, if the first day of your last menstrual period was 12th June, to work out your EDD you would have to take away 3 months (which takes you to 12th March) and then add on 7 days. This gives you an estimated due date of 19th March the following year.

How reliable is this?

While this will give you a good idea of the time around which to expect your baby's entrance into the world its not a definitive date as, because of several assumptions made, there is a large margin of error so I wouldn't go putting any money on it yet.

Firstly, the Nagele rule works on the assumption of a 28 day menstrual cycle with ovulation on day 14; if your typical cycle is, on average, shorter or longer than this, or you do not ovulate exactly 14 days from the start of your cycle (few women actually fit this calculated norm) then the EDD will be less precise and is likely to be out. Additionally, the rule also assumes a 40 week gestational period, although you are not technically pregnant for the first two weeks as ovulation and therefore conception has not yet occurred. Again this gestational period is an average based on Nagele's calculations back in the 1850's and is liable to vary. Also, because of enhanced prenatal care and furthered medical knowledge there is a developing trend towards women carrying their baby's for longer, which again may mean you overshoot the mark a little.

For women who are unsure of the date of their LMP or who suffer from irregular cycles and so are unable to use the Nagele rule, doctors are able to use ultrasound to work out an EDD. Most women will undergo examinations and ultrasounds throughout their pregnancy too and these can be more accurate in predicting length of gestation as they take into account factors such as uterus size and foetal development.

What factors influence the due date?

Although there is surprisingly little research into this area, Robert Mittendord of the University of Chicago Medical Centre identified 16 factors thought to play a causal role in determining whether a baby is born on its predicted due date. For instance, he found that Caucasian women tend to have significantly longer gestation periods than their African or Asian counterparts, with an extra 8 days of pregnancy being quite regular. Additionally, women expecting their first child tend to have significantly longer pregnancies than those expecting their second or subsequent children. Other factors such as age and prenatal diet also seem to play a role.

Is there any way I can know for sure when my baby will arrive?

Unfortunately, at the moment although we can make educated guesses as to when babies are likely to arrive, we have little way of knowing exactly when to expect them. In fact, research shows that only 5% of babies are actually born on their predicted due date, 70% tend to arrive after this whereas 30% come into the world 'early'. As a little reassurance though around 80% of healthy babies are born within the two weeks before, or the two weeks after their EDD, so mentally preparing yourself for your baby's arrival sometime between the 38th and 42nd weeks of your pregnancy is a good idea. In fact some doctors have even recommend estimating a month of arrival rather than a specific day.

It can be hard not to get too attached to the due date specified by your doctor or midwife as it is only an estimate and they themselves may alter it over the course of your pregnancy as they monitor your baby's development. Some women do find it quite stressful if their due date comes and goes without a peep from baby and this isn't usually helped by the onslaught of 'when's you're baby due?' questions that will no doubt follow you throughout your pregnancy. However, by preparing yourself well in advance (so if baby does decide to make an early show you have everything sorted), keeping busy, spending time looking after your body and enjoying some 'me' time before your baby's arrival you'll be able to take it all in your stride even if your EDD (and your baby's sense of time keeping) is a little off!

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Hie there,I'm confused becz I don't kno when exactly I got prrgnannt had my last period 14april and had unprotected sex 29 april.nw wat confuse me is that I've been given four different due dates 31,28,27,18 January so I don't kno which one to hold on baby is already in a head down position,so wen wil. I expect my little one plz,plz help
by niang009 20th Jun 2013, 3:38pm
Hi there! (second baby)
At 32wks my baby was measured at 30cm, i'm now 33wks + 2days and i read that the average size for a baby is about 42cm.

Will my baby be ok, someone please put my mind at rest as i'm constantly worrying and i know that is good for the baby. i just want to know my little girl will be ok.

please get back to me. concerned mother :(
by prilly 2nd Sep 2010, 10:53am
Kikib78.. You don't concieve till around 2 weeks after your last period begun.. so your baby will have been concieved around the time you have worked out yet the due date is calculated as the first day of your last period as that's when your egg was released.
Hope this helps coz i've just wrote it out twice trying to make it not sound gobbledygoop x
by Danni1991 1st Sep 2010, 10:04am
i'm 34 weeks pregnant with my second child and my first child was 4 weeks early (waters broke) , what are the chances of this one being that early??
by lisalefteye20 23rd Aug 2010, 10:26am
I'm really confused!!! We've just had our first scan and we were told that our baby 13 weeks and 5 days old so our EDD is 12 Dec. This is the same date we calculated with the doctor using the first day of my last period. However we were convinced the scan wud give us a due date 1-2 weeks later as we didn't have sex till that time after my period started. How can the baby measure 13 weeks and 5 days old when we know that it couldn't have been conceived anythin more than 12 weeks ago??? Has any one else had this? Maybe it's common? Perhaps conception date isn't what it goes by anyhow and the 1st day of your last period is, even with the scan? But that's just too odd to understand. Please help?
by Kikib78 12th Aug 2010, 9:06am
hi there i am only 11 weeks now with my second child but iam worried because my first child went 2weeks overdue soo i went in to get induced 8 times but nothing 5days after that they ended up breaking my waters then another 14 hours later baby stopped breathing soo i went down for a ceserrean i had a really bad labour with the first one and wonder if iam most likely bound to have the same experiance with the second baby! and can i avoid having a ceserrean cos i would really want to do it natural birth ??
by nikki101206 14th May 2010, 4:59pm
I'm 27 weeks and my midwive measured me as 24cm, anybody know if you can work out the weight of the baby through the measurements ?? Emily x
by Mazey 14th May 2010, 3:48pm
I am 35 weeks pregnant with my 5th child, I seem to remember reading that the more children you have the more likely they are to arrive early, has anyone else heard of this?
by MrsBloo 14th May 2010, 3:14pm
i am 7 weeks away from my due date and with my first daughter i had no contractions at all at any point in my labour, the hospital had to tell me when they were happening cos i could only feel pain in my hips and i have been having bad stomach pains today and my midwife has said that she is very big and could come early!! what do contractions feel like?????
by hevvafevva1980 27th Jan 2010, 9:33am
i am 39 weeks and 2 days, and had 6 shows and very bad period pains for 2 days now... how long till my baby biy wants to arrive?
by jayneelouise 11th Jan 2010, 3:11pm
I have done the estimated trick myself and it tells me 19th jan but i have been told it would be 16th jan. I suppose i am just wondering when i should have my bag for the hospital packed. I still dont completely know what to put in the bag for myself.
by rangel21 2nd Nov 2009, 10:33am
i am scared to give birth because i might have to have her out on my 34 weeks but i am really so scared because she will might be here early and i am 17 years old and iv got a little girl and i am so worryed about this but not much and right now i know that she is moving and that
by CarrieAnneC16 18th Aug 2009, 9:48am
im 37 weeks and am really anxious to get the baby out so i can hold her and stop worrying. Every morning from about 9am till 1pm the baby doesnt move and i worry myself sick.. Is there any way of speeding the waiting process up or knowing when your about to go into labour??
by Jessicasheridan 6th Aug 2009, 10:35am
I am finding very stressfull. My due date was 22 july nothing is happinning.
by davidturner1 23rd Jul 2009, 9:09am
just because your measuring further than you actually are it doesn't mean you will have your babies early. at 28 weeks i was measuring 31cm + im not 2 days overdue. measurments don't really have anything to do with how early/late your baby will be.
by sianeli 15th Jul 2009, 11:49am
My GP and midwife measured me at 34 weeks when my scan says im 31 weeks, does this mean my baby will be early?
by sms 12th Mar 2009, 9:07am
i have 4 children and im pregnant with my 5th! my 1st 2 children were both 3 days late my last 2 children were on due dates 1 by c section. does any one have any idea wen this ones likely to arrive????
by nickyhefford 5th Jan 2009, 9:26am
can you have a c-section on a due date?
by cookie3230 26th Nov 2008, 9:12am
hi every i had my first child 3 weeks early and my second child 10 wks early will i have my 3 child early ???????
by nic244 11th Nov 2008, 9:53am
Hi MussyandJen, no this doesn't mean baby will be early. You are just mesasuring slightly bigger than average. Im due on 23rd November and im measuring only 28 weeks, meaning baby is smaller than average. But don't worry yourself, if your scan is showing babys growth is normal then stick to that as its more accurate and reasurring for yourslef. Dont forget as baby's head moves futhur down into your pelvis, you will be measuring smaller. Hope this helps and good luck. x
by moopayne 10th Nov 2008, 9:53am
My midwife measured my bump and said that i was 35 weeks instead of 33, does this mean that i will be 2 weeks early??
by MussyandJenn 20th Oct 2008, 11:00am
My two children were classed as late, both arrived 3 days after original due date, (which kept being changed)
I think you should be ready from 36 weeks, but have things planned up to 42 weeks because the last few weeks go really slowly! Use this time to make the most of your sleep!
by alady 14th Aug 2008, 8:52am