Duration of Pregnancy

Duration of Pregnancy

Normal Pregnancy

The period of gestation is 266 days from the date of conception. This is simply an average. The duration of pregnancy, on the other hand, is calculated as being 280 days from the first day of the last normal regular menstrual period which is the same as ten lunar months, or forty weeks. It has always been stated that pregnancy lasts for nine months. This is not exactly true because pregnancy usually lasts for nine calendar months and seven days. Because of the confusion that arises when people try to interpret forty weeks as being nine months, it is usual for doctors and mid wives to discuss the duration of pregnancy in weeks and never in months. The same principle has been followed throughout this book.

Duration of Pregnancy

Calculation of the expected date of delivery is always from the first day of the last period and is extremely simple. If three calendar months are subtracted from the first day of the last period and seven days then added the date arrived at will be 280 days from the first day of the last period. In other words, if the first day of the last period is on June 6, the subtraction of three months goes back May, April, March 6 and then seven days added brings the expected date of delivery to March 13. Similarly, if the first day of the last period is February 20, taking off three months January, December, November 20 and adding seven days is November 27, which is the expected date of delivery.

The expected date of delivery is an average over a large number of patients. There is no guarantee that any woman will deliver on the 280th day after the first day of her last period, and in fact her chances of doing so are approximately 5 per cent. The chances of delivering on any day in the two weeks before or in the two weeks after that date are approximately 85 per cent.

If the normal menstrual cycle is less than 28 days then the expected date of confinement will be a few days earlier, whereas if the normal menstrual cycle is longer than 28 days the expected date of confinement will be later. This is because ovulation occurs early in a short menstrual cycle and late in a long menstrual cycle. The expected date of delivery will, therefore, be seven days earlier if the normal menstrual cycle is 21 days and will be seven days later if the normal cycle is 35 days, or even fourteen days later if the average normal menstrual cycle is 42 days.

It is important to emphasize that the expected date of confinement is an expected day and is only worked out as an average. Women become somewhat impatient towards the end of pregnancy and look forward with eager anticipation to the expected date of their confinement. They are often greatly disappointed when the expected day arrives and passes and there are still no signs of labour, especially when they repeatedly receive anxious enquiries from well-meaning friends. Women should not set their hearts on delivery by a certain date, but should set their sights on delivery within either two weeks before or two weeks after the expected date so that they avoid this intense disappointment.

It is interesting that age, height, size, colour, race or climate does not exert any influence upon the duration of pregnancy. Multiple pregnancies do on average deliver earlier than single pregnancies, and the actual average duration of a twin pregnancy is 37 weeks.

Accessory Methods of Calculating the Date of Delivery

Numerous precautions are taken throughout pregnancy to check that the expected date of delivery has been correctly estimated. At the first examination the size of the uterus is assessed to ensure that this agrees with the date of the last period. The foetal heart may be heard by an ultrasonic detector as early as the 12th week of pregnancy. Foetal movements are felt by the pregnant woman between the 18th and 20th week in her first pregnancy, and between the 16th and 18th week in subsequent pregnancies. The foetal heart can be heard with an ordinary stethoscope at the 24th week of pregnancy. Throughout pregnancy the size of the uterus is carefully assessed, noted and compared with the expected date of delivery.

These and many other small factors all contribute towards confirming the correct date of delivery. An X-ray examination or an ultrasonic scan can be performed if there is any real doubt about the maturity of the foetus, but this is not often necessary in someone who has attended regularly for antenatal examination. An ultrasonic scan gives accurate details of the baby’s size and maturity.

Amniocentesis will indicate the maturity of the baby. The cells in the amniotic fluid are stained to show the exact duration of pregnancy. This method is not used unless there is some anxiety about the condition of the baby.

Prolonged Pregnancy

Prolonged pregnancy is one which continues more than 14 days past the expected date of confinement. The labour and the baby are then considered postmature.

Occasionally prolonged pregnancy becomes a legal issue when paternity is disputed, and the longest pregnancy accepted in the British courts of law has been over 340 days. Many pregnancies of over 320 days are recorded but really lorig pregnancies are usually of normal duration, having been conceived after some months of non-pregnant amenorrhoea.

The reason for the onset of labour is not known, but in order to try and understand how and why postmaturity affects the baby, it is best to consider the placenta and the baby together. The placenta, or afterbirth, reaches maturity at about the 32nd or 34th week of pregnancy, and from this date onwards it gradually becomes less efficient. At the same time the demands on the placenta are increasing because each day the growing baby requires more and more oxygen, and its needs also become more complicated with the baby’s continued maturation and development. At the 32nd week of pregnancy there is an enormous reserve in the placenta, but as the placenta ages and the baby grows the reserve is slowly reduced. At 40 weeks the baby has grown to its fullest extent and the placenta still contains sufficient reserve to withstand the trials of labour.


From the 40th week onwards the baby seems to realize that it is on a losing wicket. It is doubtful if it increases in size and it certainly begins to reduce some of its requirements. The fat which it has stored beneath its skin is gradually used and although the baby’s brain, liver, lungs and heart continue to mature and its bones continue to get harder, it may in fact actually lose weight. By the 42nd week the baby will have very little fat beneath its skin and by the 43 rd week probably none at all, so that if delivered the baby will look thin, rather scraggy, dry and rather wrinkled. The fingernails will be long. Desquamation, or peeling, of the superficial layers of the skin will be obvious over the hands and feet and, within a day or two of birth, over the skin creases of the knees, elbows, wrists and groins. These postmature changes in the baby are designed to conserve its resources and not place undue demands upon the continuously ageing placenta. Once it is delivered the child will rapidly replace its subcutaneous fat and Within a few days will look like any normal baby.

Opinions are divided on the possible dangers of postmaturity. Most obstetricians would agree that patients with pre-eclampsia should not be allowed to go past their expected date. It may also be inadvisable to allow women over the age of 35 who are expecting their first child to go much beyond their expected date. Most obstetricians, however, would agree that providing everything else is normal there is no harm in allowing a normal pregnancy to run into the second week past the expected date. It is when the 42nd week is reached that clearly some division of opinion arises. Some obstetricians consider that most patients should be induced when they reach the 42nd week of pregnancy, while others consider that there is little or no indication to perform induction of labour before the 43 rd or even the 44th week. It is emphasized that each individual pregnancy is treated on its merits and no hard and fast rule can be laid down.

Milestones of Pregnancy

This is a short summary of some of the main symptoms and signs that occur during pregnancy and enable a woman, and her doctor and midwife, to confirm the duration of pregnancy and, also the expected date of arrival of her baby. Each point is discussed at greater length elsewhere.

Amenorrhoea. The absence of a menstrual period is the first and the most classic sign of the onset of pregnancy. Occasionally a so-called ‘partially suppressed period’ may occur in which the menstrual loss is very much shorter and very much less than normal.

Breast changes. The onset of breast changes varies in its timing, but many women notice changes as soon as they miss their first period. These may be very slight initially but they gradually become more noticeable.

Nausea may commence at any stage during the first three months of pregnancy but frequently begins one or two weeks after the first period has been missed. In fact actual nausea may be fairly late in its onset and may be preceded by lack of appetite, a disinterest in food, or even an active dislike of other things such as smoking or alcohol. Nausea need not necessarily occur in the morning. It can occur at any time during the day or evening.

Frequency of micturition. Increase in frequency of micturition is a common early symptom of pregnancy, usually beginning about the 6th week. An excessive desire to pass urine may occur during the day or at night.

Uterine size. The uterus is enlarged into the abdomen by the 12th week of pregnancy and is easily palpable. From this time on the size of the uterus will follow a characteristic pattern and will enlarge steadily and proportionately as the pregnancy progresses. It normally reaches up to the margin of the rib cage in the mid-line at the 36th week.

Relief of nausea. The nausea, sickness, tiredness and other trying symptoms of early pregnancy usually disappear spontaneously by the end of the 14th week. This is a most characteristic date.

Pigmentation of the breasts, together with the formation of the linea nigra (the black line down the centre of the abdomen) and darkening of moles and freckles, will commence shortly after the end of the 14th week.

Foetal heart. The foetal heart can usually be heard by ultrasonic apparatus between the 12th and 14th week and by a stethoscope at about the 24th week of pregnancy.

Foetal movements. There is no hard and fast rule about the first sensations of foetal movements, but generally movements of a first baby are felt between the 18th and 20th week, and between the 16th and 18th week in a subsequent pregnancy.

Lightening. In a first pregnancy the baby’s head drops down into the pelvis at about the 36th week. This results in considerable relief from pressure in the upper abdomen and is known as lightening. When it occurs it is quite characteristic.

Clinical examination. Frequent clinical examinations throughout pregnancy, carefully noting the size of the uterus and the size of the baby, together with all the above factors, confirm the duration of pregnancy and the expected date of delivery.