Length of pregnancy can vary by up to five weeks, scientists discover

 

The length of a pregnancy naturally varies between different women by as much as five weeks, scientists have discovered for the first time.

Although it has always been known that human pregnancies usually last between 37 and 42 weeks, the variation has previously been attributed to imperfect methods of estimating due dates.

A woman's due date is calculated as 280 days after the onset of her last menstruation and is only ever considered a best guess. Only four per cent of women deliver on their due date and only 70 per cent within 10 days of it.

However researchers from the National Institute of Environmental Health Sciences (NIEHS) were able to pinpoint the moment women became pregnant by taking daily urine samples - enabling them to isolate the role that natural variation plays in pregnancy length.

In a group of 125 women, they found the average time from ovulation to birth was 268 days - 38 weeks and two days - and that the length of pregnancies varied by as much as 37 days, according to the research, published today in the journal Human Reproduction.

“We were a bit surprised by this finding,” said Dr Anne Marie Jukic, a postdoctoral at the Epidemiology Branch at the NIEHS. “We know that length of gestation varies among women, but some part of that variation has always been attributed to errors in the assignment of gestational age. Our measure of length of gestation does not include these sources of error, and yet there is still five weeks of variability. It's fascinating.”

The length of a pregnancy could be affected by a number of factors. Older women tended to deliver later, while women who had themselves been heavier at birth also had longer pregnancies.

The Royal College of Midwives (RCM) said the research reaffirmed professionals' experience “that every woman is different” and urged practitioners to be cautious intervening too early in pregnancy, but said that due dates were still useful.

Mervi Jokinen, practice and standards professional advisor at the RCM said: “I do not think that this signals the end of midwives and other health professionals giving a woman a single due date. It should be explained to the woman that the due date is always an estimate and as this research and our experience shows, this can vary widely.”

Start your day with The Independent, sign up for daily news emails
News
i100
Arts and Entertainment
Billie Piper as Brona in Penny Dreadful
tvReview: It’s business as usual in Victorian London. Let’s hope that changes as we get further into the new series spoiler alert
Life and Style
A nurse tends to a recovering patient on a general ward at The Queen Elizabeth Hospital in Birmingham
health
News
science
Arts and Entertainment
No Offence
tvReview: No Offence has characters who are larger than life and yet somehow completely true to life at the same time spoiler alert
News
Chuck Norris pictured in 1996
people
ebooks
ebooksA celebration of British elections
Arts and Entertainment
Sarah Lucas, I SCREAM DADDIO, Installation View, British Pavilion 2015
artWhy Sarah Lucas is the perfect choice to represent British art at the Venice Biennale
News
A voter placing a ballot paper in the box at a polling station
i100
News
people
Arts and Entertainment
The Queen (Kristin Scott Thomas) in The Audience
theatreReview: Stephen Daldry's direction is crisp in perfectly-timed revival
Sport
football
  • Get to the point
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

SThree: Trainee Recruitment Consultant - Dublin

£13676.46 - £16411.61 per annum + OTE: SThree: SThree Trainee Recruitment Cons...

Ashdown Group: Marketing or Business Graduate Opportunity - Norwich - £22,000

£18000 - £22000 per annum + training: Ashdown Group: Business and Marketing Gr...

SThree: Trainee Recruitment Consultant

£20000 - £25000 per annum + Commission: SThree: Are you great at building rela...

Ashdown Group: Database Analyst - Birmingham - £22,000 plus benefits

£20000 - £22000 per annum + excellent benefits: Ashdown Group: Application Sup...

Day In a Page

General Election 2015: Ed Miliband's unlikely journey from hapless geek to heart-throb

Miliband's unlikely journey from hapless geek to heart-throb

He was meant to be Labour's biggest handicap - but has become almost an asset
General Election 2015: A guide to the smaller parties, from the the National Health Action Party to the Church of the Militant Elvis Party

On the margins

From Militant Elvis to Women's Equality: a guide to the underdogs standing in the election
Amr Darrag: Ex-Muslim Brotherhood minister in exile still believes Egypt's military regime can be replaced with 'moderate' Islamic rule

'This is the battle of young Egypt for the future of our country'

Ex-Muslim Brotherhood minister Amr Darrag still believes the opposition can rid Egypt of its military regime and replace it with 'moderate' Islamic rule, he tells Robert Fisk
Why patients must rely less on doctors: Improving our own health is the 'blockbuster drug of the century'

Why patients must rely less on doctors

Improving our own health is the 'blockbuster drug of the century'
Sarah Lucas is the perfect artist to represent Britain at the Venice Biennale

Flesh in Venice

Sarah Lucas has filled the British pavilion at the Venice Biennale with slinky cats and casts of her female friends' private parts. It makes you proud to be a woman, says Karen Wright
11 best anti-ageing day creams

11 best anti-ageing day creams

Slow down the ageing process with one of these high-performance, hardworking anti-agers
Juventus 2 Real Madrid 1: Five things we learnt, including Iker Casillas is past it and Carlos Tevez remains effective

Juventus vs Real Madrid

Five things we learnt from the Italian's Champions League first leg win over the Spanish giants
Ashes 2015: Test series looks a lost cause for England... whoever takes over as ECB director of cricket

Ashes series looks a lost cause for England...

Whoever takes over as ECB director of cricket, says Stephen Brenkley
Fishing for votes with Nigel Farage: The Ukip leader shows how he can work an audience as he casts his line to the disaffected of Grimsby

Fishing is on Nigel Farage's mind

Ukip leader casts a line to the disaffected
Who is bombing whom in the Middle East? It's amazing they don't all hit each other

Who is bombing whom in the Middle East?

Robert Fisk untangles the countries and factions
China's influence on fashion: At the top of the game both creatively and commercially

China's influence on fashion

At the top of the game both creatively and commercially
Lord O’Donnell: Former cabinet secretary on the election and life away from the levers of power

The man known as GOD has a reputation for getting the job done

Lord O'Donnell's three principles of rule
Rainbow shades: It's all bright on the night

Rainbow shades

It's all bright on the night
'It was first time I had ever tasted chocolate. I kept a piece, and when Amsterdam was liberated, I gave it to the first Allied soldier I saw'

Bread from heaven

Dutch survivors thank RAF for World War II drop that saved millions
Britain will be 'run for the wealthy and powerful' if Tories retain power - Labour

How 'the Axe' helped Labour

UK will be 'run for the wealthy and powerful' if Tories retain power