How long is the pregnancy

The length of a normal pregnancy can be anything from 37 weeks to 42 weeks. So you have a five-week period when your baby could arrive at any time and it would be considered normal.

If you give birth before 37 weeks of pregnancy, your baby is defined as premature and is likely to need extra care. A pregnancy that lasts longer than 42 weeks is called a prolonged pregnancy and may carry a higher risk of complications.

Inhaltsverzeichnis Show

Length of gestation
Pregnancy ultrasound
The diagnostic uses of pregnancy ultrasound
Premature babies
Overdue babies
Where to get help
Things to remember

Even though all pregnant women are given a due date or estimated date of delivery (EDD), only about one in 25 babies are born on their exact due date.

Most babies arrive between 37 weeks and 41 weeks of pregnancy, usually within a week either side of their due date. Twins and particularly triplets are more likely to be born premature.

The due date you are given at your dating scan will be more accurate than using your last period to calculate it. The day of ovulation in the menstrual cycle can vary from woman to woman depending on their cycle length, while a scan can pinpoint the exact week and day of pregnancy based on the size of the embryo.

The length of your pregnancy may also be influenced by how long your baby took to implant when she was conceived. Fertilised eggs that take longer to implant tend to have a longer gestation from implantation to birth.

Mothers with naturally longer pregnancies also tend to be older, have had longer pregnancies in previous births and have been heavier at birth than mothers with shorter pregnancies.

Tall women tend to have longer pregnancies than short women, and white European women tend to have naturally longer pregnancies than African-Caribbean or Asian women. It's common for babies to arrive a week or so after their due date. About one in five babies are born at 41 weeks of pregnancy or more.

It's thought that between one in 10 and one in 20 women has a pregnancy that would naturally last longer than 42 weeks. Although most women in the UK have their labour induced before they get to this point.

Page last reviewed: 29 April 2021
Next review due: 29 April 2024

Page last reviewed: 13 October 2021
Next review due: 13 October 2024

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The unborn baby spends around 38 weeks in the uterus, but the average length of pregnancy, or gestation, is counted at 40 weeks.
Pregnancy is counted from the first day of the woman’s last period, not the date of conception which generally occurs two weeks later.
Since some women are unsure of the date of their last menstruation (perhaps due to period irregularities), a baby is considered full term if its birth falls between 37 to 42 weeks of the estimated last menstruation date.

The unborn baby spends around 37 weeks in the uterus (womb), but the average length of pregnancy, or gestation, is calculated as 40 weeks. This is because pregnancy is counted from the first day of the woman’s last period, not the date of conception which generally occurs two weeks later, followed by five to seven days before it settles in the uterus. Since some women are unsure of the date of their last menstruation (perhaps due to period irregularities), a pregnancy is considered full term if birth falls between 37 to 42 weeks of the estimated last menstruation date.

A baby born prior to week 37 is considered premature, while a baby that still hasn’t been born by week 42 is said to be overdue. In many cases, labour will be induced in the case of an overdue baby.

Length of gestation

The average length of human gestation is 280 days, or 40 weeks, from the first day of the woman’s last menstrual period. The medical term for the due date is estimated date of confinement (EDC). However, only about four per cent of women actually give birth on their EDC. There are many online pregnancy calculators (see Baby due date calculator that can tell you when your baby is due, if you type in the date of the first day of your last period.

A simple method to calculate the due date is to add seven days to the date of the first day of your last period, then add nine months. For example, if the first day of your last period was 1 February, add seven days (8 February) then add nine months, for a due date of 8 November.

Irregular menstrual cycles can mean that some women aren’t sure of when they conceived. Some clues to the length of gestation include:

Ultrasound examination (especially when performed between six and 12 weeks)
Size of uterus on vaginal or abdominal examination
The time fetal movements are first felt (an approximate guide only).

Pregnancy ultrasound

A pregnancy ultrasound is a non-invasive test that scans the unborn baby and the mother’s reproductive organs using high frequency sound waves. The general procedure for a pregnancy ultrasound includes:

The woman lies on a table.
A small amount of a clear, conductive jelly is smeared on the woman’s abdomen.
The operator places the small hand-held instrument called a transducer onto the woman’s abdomen.
The transducer is moved across the abdomen. The sound waves bounce off internal structures (including the baby) and are transmitted back to the transducer. The sound waves are then translated into a two-dimensional picture on a monitor. The mother doesn’t feel or hear the transmission of the sound waves.
By measuring the baby’s body parts, such as head circumference and the length of long bones, the operator can estimate its gestational age.

The diagnostic uses of pregnancy ultrasound

Apart from helping to pinpoint the unborn baby’s due date, pregnancy ultrasounds are used to diagnose a number of conditions including:

Multiple fetuses
Health problems with the baby
Ectopic pregnancy (the embryo lodges in the fallopian tube instead of the uterus)
Abnormalities of the placenta such as placenta praevia, where the placenta is positioned over the neck of the womb (cervix)
The health of the mother’s reproductive organs.

Premature babies

A baby born prior to week 37 is considered premature. The odds of survival depend on the baby’s degree of prematurity. The closer to term (estimated date of confinement, or EDC) the baby is born, the higher its chances of survival - after 34 weeks gestation with good paediatric care almost all babies will survive. Premature babies are often afflicted by various health problems, caused by immature internal organs. Respiratory difficulties and an increased susceptibility to infection are common.

Often there is no known cause for a premature labour; however, some of the maternal risk factors may include:

Drinking alcohol or smoking during pregnancy
Low body weight prior to pregnancy
Inadequate weight gain during pregnancy
No prenatal care
Emotional stress
Placenta problems such as placenta praevia
Various diseases such as diabetes and congestive heart failure
Infections such as syphilis.

Overdue babies

Around five out of every 100 babies will be overdue, or more than 42 weeks gestation. If you have gone one week past your due date without any signs of impending labour, your doctor will want to closely monitor your condition. Tests include:

Monitoring the fetal heart rate
Using a cardiotocograph machine
Performing ultrasound scans.

The placenta starts to deteriorate after 38 weeks or so, which means an overdue baby may not get enough oxygen. An overdue baby could also grow too large for vaginal delivery. Generally, an overdue baby will be induced once it is two weeks past its expected date. Some of the methods of induction include:

Vaginal prostaglandin gel - to help dilate the cervix
Amniotomy - breaking the waters, sometimes called an artificial rupture of membranes (ARM)
Oxytocin - a synthetic form of this hormone is given intravenously to stimulate uterine contractions.

Where to get help

Your doctor
Your obstetrician
Midwife or childbirth educator

Things to remember

The unborn baby spends around 38 weeks in the uterus, but the average length of pregnancy, or gestation, is counted at 40 weeks.
Pregnancy is counted from the first day of the woman’s last period, not the date of conception which generally occurs two weeks later.
Since some women are unsure of the date of their last menstruation (perhaps due to period irregularities), a baby is considered full term if its birth falls between 37 to 42 weeks of its estimated due date.

Common concerns and discomforts: overdue baby, Mother’s Bliss, UK.
Going Overdue, 2001, Centre for Reproduction and Minimally Invasive Surgery.
Premmie-L FAQ and advice sheets, Parents of Premature Babies Inc.(Preemie-L). More information here.
Pregnancy: what to expect when it’s past your due date, Family Doctor, USA.
Ultrasound, Women Health Information, Royal Women’s Hospital, Melbourne. More information here .

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