What is the gestational age? How to calculate the correct number of weeks and expected delivery date【supervised by doctor】

How do you know the number of weeks of pregnancy and the date of delivery, which can be easily mistaken?

When you find out you are pregnant, the first thing you probably wonder is how many months and weeks you are.

At the same time, you also want to know exactly when the baby is due.

In Japan, there is a long-standing tradition of calculating the number of weeks of pregnancy as 10 months and 10 days.

However, for example, if the due date is 10 October, it does not mean that the pregnancy is 1 January.

The actual period between pregnancy and the expected date of delivery is not 10 months and 10 days, but 280 days (40 weeks and 0 day) is considered the standard number of days.

In this article, we will tell you ‘why it is easy to make mistakes in calculating the expected date of delivery’ and ‘how to calculate the number of weeks of pregnancy and the date of delivery to avoid mistakes’.

‘Post-menstrual fetal age’ and ‘menstrual cycle’ important for calculating the number of weeks of pregnancy

Post-fertilisation and post-menstrual age

There are two ways of thinking about calculating the number of weeks of pregnancy: ‘post-fetal age after fertilisation’ and ‘post-menstrual age’.

Post-fetal age after conception is calculated with the day of conception as day 1 of the gestational period and expressed in full days and full weeks.

Post-menstrual fetal age, on the other hand, counts the gestational period from when the last menstrual period begins.

The day on which the last menstrual period began is counted as day 0 and expressed in full days and full weeks.

The ‘post-fetal age after fertilisation’ can also be expressed by subtracting two weeks from the ‘post-menstrual age’.

As it is difficult to pinpoint the exact date of fertilisation or the date of implantation of the fertilised egg, the ‘post-menstrual fetal age’ calculation method is generally used in Japan today, as it is in the USA.

Menstrual cycle

The ‘menstrual cycle’ is an important factor in counting the number of months or weeks of pregnancy.

The menstrual cycle varies from person to person, but it is generally considered to be 28 days, with a period of between 25 and 38 days being within the normal range.

The gestational age according to the ‘post-menstrual fetal age’ counting system is 280 days (10 months), counting 28 days from the start of the last menstrual period as one month.

For example, if the last menstrual period before conception started on 1 April, the expected date of delivery would be 280 days later, on 6 January the following year.

However, as we mentioned earlier, there are individual differences in the menstrual cycle, which can cause deviations here.

If the last menstrual period is on 1 April, the due date is 3 January for those with a 25-day cycle and 10 January for those with a 32-day cycle.

Even for those who normally have a 28-day menstrual cycle, if the day of ovulation is shifted by one week due to physical condition or stress, the date of delivery may be earlier or later than 280 days later, depending on the date of the shift.

280 days (40 weeks 0 days) = gestational age for those with a menstrual cycle of 28 days

From the above, it can be seen that the current standard gestation period in Japan, 280 days (40 weeks and 0 day), is the gestation period of a woman with a menstrual cycle of 28 days.

What is the expected delivery date in the case of artificial insemination and IVF?

In artificial insemination and in vitro fertilisation, the date of egg retrieval or embryo transfer from the mother is considered to be week 2, day 0, and 266 days later is calculated as the delivery date.

Even in such cases, the original expected delivery date may change depending on the development and condition of the baby.

Naegele Approximation Method

The Negele Approximation Method is an easy way for those with a 28-day menstrual cycle to predict the expected date of their baby’s birth from the start date of their last menstrual period.

The Naegele Approximation Method calculates the expected date of birth by applying the day of the month when the last menstrual period started to the following formula.

Expected month of delivery = month of last menstrual period – 3 (add 9 if not subtracted)

Expected date of delivery = date last menstrual period started + 7

If the last menstrual period started on 5 January, the expected delivery date is October (1+9)12 (5+7)

If the last menstrual period started on 10 November, the expected delivery date is 17 August (11-3) (10 + 7), meaning that the gestation period is from 0 weeks and 0 days on 10 November to 17 August, 40 weeks (280 days) later.

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Actual number of weeks of childbirth

About 61% of all women give birth at 37-39 weeks

The standard gestation period is 40 weeks from the start of the last menstrual period to the date of delivery, but in reality less than 20% of all babies are delivered at 40 weeks.

The following table lists the number of weeks of delivery, based on the ‘Report Distribution of weeks of childbirth’ of the Perinatal Committee of the Japanese Society of Obstetrics and Gynaecology, published in June 2021.

Number of weeks of childbirth	Number of case	Percentage of total（％)
22～34	14,934	6.4
35	6,107	2.6
36	10,923	4.7
37	33,053	14.1
38	54,386	23.3
39	53,316	22.8
40	45,376	19.4
41	15,322	6.6
42～	351	0.15

As can be seen, the largest proportion of births actually take place at 38 weeks (23.3%), with approximately 61% of all births taking place before 40 weeks (37-39 weeks).

In addition, nearly 7% of women give birth after 40 weeks. 40 weeks is only a guide, and knowing your ovulation date and menstrual cycle is the basis for more accurate calculation of the expected date of delivery and gestational age.

Positive term, premature and perterm births

Births are divided into full-term, premature and perterm births, depending on the stage of gestation at which delivery occurred.

• 【Positive term birth】Births between 37 weeks 0 days and 41 weeks 6 days. The foetus is fully developed and there are no obstacles to postnatal development at any time during this period.
• 【Premature birth】Birth between 22 weeks 0 days and 36 weeks 6 days. The foetus may be underdeveloped and there is a risk of infection and morbidity due to insufficient immune function and thermoregulation after birth.
• 【Perterm birth】Delivery after 42 weeks 0 days gestation. There is a risk of reduced placental function and an increased risk of placental aspiration syndrome, which may lead to a giant baby and breathing difficulties as the foetus inhales sterile stools into the lungs.

The expected date of childbirth is fixed【by 12 weeks’ gestation】

The expected delivery date at the time of pregnancy discovery often changes as the weeks of pregnancy progress, for example, if the menstrual cycle is unclear or the cycle itself is irregular.

In most cases, the expected date of delivery is predicted by measuring the foetal head length (the length of the foetus from the tip of the head to the buttocks) by ultrasound at 8-11 weeks’ gestation.

By 12 weeks’ gestation, the expected date of delivery is fixed.

Length of pregnancy and pregnancy tests and NIPT

Pregnancy occurs in three stages: the encounter between the sperm and the ovulated egg, the fertilisation of the egg and implantation.

These three processes are collectively known as conception.

The 10-month gestation period is divided into the following categories for each of the three months elapsed.

• 【Early pregnancy】Up to 13 weeks 6 days… Once fertilisation is established after intercourse, the fertilised egg is implanted in about 7 days… Between 4 and 12 weeks, the baby’s internal organs and tissues are formed.
• 【Mid-pregnancy】14 weeks 0 days to 27 weeks 6 days… This is the period known as the stable period. The size of your abdomen is now clearly visible and you are recognised as an expectant mother by those around you. You will feel frequent fetal movements.
• 【Last trimester of pregnancy】28 weeks to … In the last trimester, the baby’s basic body and functions are in place and the baby is ready to be born at any time. It is also after 36 weeks in the second trimester that it is generally referred to as the last month.

The risk of developing symptoms of ‘gestational diabetes’ increases during this period, so care must be taken to manage the diet.

A definitive diagnosis of placenta previa, where the placenta is lower than normal and covers the uterine outlet, is also made by the end of the second trimester, around 31 weeks.

In most cases of placenta previa, a caesarean section is performed.

When can pregnancy tests be used?

It can be used from one week after the expected start of menstruation.

Use of the test before that time may result in a positive result, but it is also possible to get a positive result in rare cases due to causes other than pregnancy (flying test), so make sure to use the test in accordance with the time of use.

If a positive test occurs, see a gynaecologist as soon as possible.

NIPT

Prenatal diagnosis involves testing a pregnant woman’s blood to check for abnormalities in the baby’s DNA.

By testing and understanding the health of the baby before birth, treatment (medication) can be given at the prenatal stage or preparations can be made to start treatment immediately after the birth.

First births at an age that used to be known as ‘old age’ are nowadays common, and more and more people are requesting prenatal diagnosis in order to be fully prepared for the birth of their child.

NIPT can be performed once pregnancy is confirmed by an ultrasound scan.

Although the test is highly accurate, an amniotic fluid test is then required to make a definitive diagnosis. An amniotic fluid test can be performed from 15 weeks of pregnancy onwards.

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Examples of how the expected date of childbirth can be incorrectly calculated

The day of intercourse or the day of conception is counted as 0 weeks and 0 days

A frequent case of incorrectly calculating the expected date of delivery is the perception that 0 weeks 0 days of pregnancy = the day of intercourse or the day of conception.

As it is difficult to ascertain when conception occurred, the start of the last menstrual period, which is easier to manage by keeping a regular record of basal body temperature, physical condition and menstrual cycle, is taken as 0 weeks 0 days of pregnancy.

Unclear perception of the menstrual cycle due to irregular menstruation or poor management of basal body temperature

Even if you know that the start of the last menstrual period is 0 weeks 0 days of pregnancy, there are often errors in the expected date of delivery if the menstrual cycle is not well understood, for example due to irregular menstruation or lack of control of physical condition and basal body temperature.

It is very important to take care of your health even before the time of conception in order to know the expected date of delivery more accurately and, if you wish to have a prenatal check-up, to choose a time when both mother and child will be comfortable.

Check the calendar for expected childbirth dates

So far, we have provided you with information on how to calculate the correct due date and gestational age, which can easily be mistaken.

Now, we will conclude with a clear calendar listing the expected date of delivery.

It is based on the date of the start of the last menstrual period. It is also possible to calculate the approximate date of ovulation by working backwards from the expected date of delivery to the last menstrual period.

Under each last menstrual start date, the corresponding expected delivery date is listed.

※This is a 28-day menstrual cycle version, so you can add or subtract days according to your menstrual cycle.

If the last menstrual period started in January

Last menstrual start date	1 Jan	2 Jan	3 Jan	4 Jan	5 Jan	6 Jan	7 Jan	8 Jan	9 Jan	10 Jan
Date of birth expected date	8 Oct	9 Oct	10 Oct	11 Oct	12 Oct	13 Oct	14 Oct	15 Oct	16 Oct	17 Oct
Last menstrual start date	11 Jan	12 Jan	13 Jan	14 Jan	15 Jan	16 Jan	17 Jan	18 Jan	19 Jan	20 Jan
Date of birth expected date	18 Oct	19 Oct	20 Oct	21 Oct	22 Oct	23 Oct	24 Oct	25 Oct	26 Oct	27 Oct
Last menstrual start date	21 Jan	22 Jan	23 Jan	24 Jan	25 Jan	26 Jan	27 Jan	28 Jan	29 Jan	30 Jan
Date of birth expected date	28 Oct	29 Oct	30 Oct	31 Oct	1 Nov	2 Nov	3 Nov	4 Nov	5 Nov	6 Nov
Last menstrual start date	31 Jan
Date of birth expected date	7 Nov

If the last menstrual period started in February

Last menstrual start date	1 Feb	2 Feb	3 Feb	4 Feb	5 Feb	6 Feb	7 Feb	8 Feb	9 Feb	10 Feb
Date of birth expected date	8 Nov	9 Nov	10 Nov	11 Nov	12 Nov	13 Nov	14 Nov	15 Nov	16 Nov	17 Nov
Last menstrual start date	11 Feb	12 Feb	13 Feb	14 Feb	15 Feb	16 Feb	17 Feb	18 Feb	19 Feb	20 Feb
Date of birth expected date	18 Nov	19 Nov	20 Nov	21 Nov	22 Nov	23 Nov	24 Nov	25 Nov	26 Nov	27 Nov
Last menstrual start date	21 Feb	22 Feb	23 Feb	24 Feb	25 Feb	26 Feb	27 Feb	28 Feb
Date of birth expected date	28 Nov	29 Nov	30 Nov	1 Dec	2 Dec	3 Dec	4 Dec	5 Dec

If the last menstrual period started in March

Last menstrual start date	1 Mar	2 Mar	3 Mar	4 Mar	5 Mar	6 Mar	7 Mar	8 Mar	9 Mar	10 Mar
Date of birth expected date	6 Dec	7 Dec	8 Dec	9 Dec	10 Dec	11 Dec	12 Dec	13 Dec	14 Dec	15 Dec
Last menstrual start date	11 Mar	12 Mar	13 Mar	14 Mar	15 Mar	16 Mar	17 Mar	18 Mar	19 Mar	20 Mar
Date of birth expected date	16 Dec	17 Dec	18 Dec	19 Dec	20 Dec	21 Dec	22 Dec	23 Dec	24 Dec	25 Dec
Last menstrual start date	21 Mar	22 Mar	23 Mar	24 Mar	25 Mar	26 Mar	27 Mar	28 Mar	29 Mar	30 Mar
Date of birth expected date	26 Dec	27 Dec	28 Dec	29 Dec	30 Dec	31 Dec	Next 1 Jan	Next 2 Jan	Next 3 Jan	Next 4 Jan
Last menstrual start date	31 Mar
Date of birth expected date	Next 5 Jan

If the last menstrual period started in April

Last menstrual start date	1 Apr	2 Apr	3 Apr	4 Apr	5 Apr	6 Apr	7 Apr	8 Apr	9 Apr	10 Apr
Date of birth expected date	Next 6 Jan	Next 7 Jan	Next 8 Jan	Next 9 Jan	Next 10 Jan	Next 11 Jan	Next 12 Jan	Next 13 Jan	Next 14 Jan	Next 15 Jan
Last menstrual start date	11 Apr	12 Apr	13 Apr	14 Apr	15 Apr	16 Apr	17 Apr	18 Apr	19 Apr	20 Apr
Date of birth expected date	Next 16 Jan	Next 17 Jan	Next 18 Jan	Next 19 Jan	Next 20 Jan	Next 21 Jan	Next 22 Jan	Next 23 Jan	Next 24 Jan	Next 25 Jan
Last menstrual start date	21 Apr	22 Apr	23 Apr	24 Apr	25 Apr	26 Apr	27 Apr	28 Apr	29 Apr	30 Apr
Date of birth expected date	Next 26 Jan	Next 27 Jan	Next 28 Jan	Next 29 Jan	Next 30 Jan	Next 31 Jan	Next 1 Feb	Next 2 Feb	Next 3 Feb	Next 4 Feb

If the last menstrual period started in May

Last menstrual start date	1 May	2 May	3 May	4 May	5 May	6 May	7 May	8 May	9 May	10 May
Date of birth expected date	Next 5 Feb	Next 6 Feb	Next 7 Feb	Next 8 Feb	Next 9 Feb	Next 10 Feb	Next 11 Feb	Next 12 Feb	Next 13 Feb	Next 14 Feb
Last menstrual start date	11 May	12 May	13 May	14 May	15 May	16 May	17 May	18 May	19 May	20 May
Date of birth expected date	Next 15 Feb	Next 16 Feb	Next 17 Feb	Next 18 Feb	Next 19 Feb	Next 20 Feb	Next 21 Feb	Next 22 Feb	Next 23 Feb	Next 24 Feb
Last menstrual start date	21 May	22 May	23 May	24 May	25 May	26 May	27 May	28 May	29 May	30 May
Date of birth expected date	Next 25 Feb	Next 26 Feb	Next 27 Feb	Next 28 Feb	Next 1 Mar	Next 2 Mar	Next 3 Mar	Next 4 Mar	Next 5 Mar	Next 6 Mar
Last menstrual start date	31 May
Date of birth expected date	Next 7 Mar

If the last menstrual period started in June

Last menstrual start date	1 Jun	2 Jun	3 Jun	4 Jun	5 Jun	6 Jun	7 Jun	8 Jun	9 Jun	10 Jun
Date of birth expected date	Next 8 Mar	Next 9 Mar	Next 10 Mar	Next 11 Mar	Next 12 Mar	Next 13 Mar	Next 14 Mar	Next 15 Mar	Next 16 Mar	Next 17 Mar
Last menstrual start date	11 Jun	12 Jun	13 Jun	14 Jun	15 Jun	16 Jun	17 Jun	18 Jun	19 Jun	20 Jun
Date of birth expected date	Next 18 Mar	Next 19 Mar	Next 20 Mar	Next 21 Mar	Next 22 Mar	Next 23 Mar	Next 24 Mar	Next 25 Mar	Next 26 Mar	Next 27 Mar
Last menstrual start date	21 Jun	22 Jun	23 Jun	24 Jun	25 Jun	26 Jun	27 Jun	28 Jun	29 Jun	30 Jun
Date of birth expected date	Next 18 Mar	Next 28 Mar	Next 29 Mar	Next 30 Mar	Next 31 Mar	Next 1 Apr	Next 2 Apr	Next 3 Apr	Next 4 Apr	Next 5 Apr

If the last menstrual period started in July

Last menstrual start date	1 Jul	2 Jul	3 Jul	4 Jul	5 Jul	6 Jul	7 Jul	8 Jul	9 Jul	10 Jul
Date of birth expected date	Next 7 Apr	Next 8 Apr	Next 9 Apr	Next 10 Apr	Next 11 Apr	Next 12 Apr	Next 13 Apr	Next 14 Apr	Next 15 Apr	Next 16 Apr
Last menstrual start date	11 Jul	12 Jul	13 Jul	14 Jul	15 Jul	16 Jul	17 Jul	18 Jul	19 Jul	20 Jul
Date of birth expected date	Next 17 Apr	Next 18 Apr	Next 19 Apr	Next 20 Apr	Next 21 Apr	Next 22 Apr	Next 23 Apr	Next 24 Apr	Next 25 Apr	Next 26 Apr
Last menstrual start date	21 Jul	22 Jul	23 Jul	24 Jul	25 Jul	26 Jul	27 Jul	28 Jul	29 Jul	30 Jul
Date of birth expected date	Next 17 Apr	Next 27 Apr	Next 28 Apr	Next 29 Apr	Next 30 Apr	Next 1 May	Next 2 May	Next 3 May	Next 4 May	Next 5 May
Last menstrual start date	31 Jul
Date of birth expected date	Next 7 May

If the last menstrual period started in August

Last menstrual start date	1 Aug	2 Aug	3 Aug	4 Aug	5 Aug	6 Aug	7 Aug	8 Aug	9 Aug	10 Aug
Date of birth expected date	Next 8 May	Next 9 May	Next 10 May	Next 11 May	Next 12 May	Next 13 May	Next 14 May	Next 15 May	Next 16 May	Next 17 May
Last menstrual start date	11 Aug	12 Aug	13 Aug	14 Aug	15 Aug	16 Aug	17 Aug	18 Aug	19 Aug	20 Aug
Date of birth expected date	Next 18 May	Next 19 May	Next 20 May	Next 21 May	Next 22 May	Next 23 May	Next 24 May	Next 25 May	Next 26 May	Next 27 May
Last menstrual start date	21 Aug	22 Aug	23 Aug	24 Aug	25 Aug	26 Aug	27 Aug	28 Aug	29 Aug	30 Aug
Date of birth expected date	Next 28 May	Next 29 May	Next 30 May	Next 31 May	Next 1 Jun	Next 2 Jun	Next 3 Jun	Next 4 Jun	Next 5 Jun	Next 6 Jun
Last menstrual start date	31 Aug
Date of birth expected date	Next 7 Jun

If the last menstrual period started in September

Last menstrual start date	1 Sep	2 Sep	3 Sep	4 Sep	5 Sep	6 Sep	7 Sep	8 Sep	9 Sep	10 Sep
Date of birth expected date	Next 8 Jun	Next 9 Jun	Next 10 Jun	Next 11 Jun	Next 12 Jun	Next 13 Jun	Next 14 Jun	Next 15 Jun	Next 16 Jun	Next 17 Jun
Last menstrual start date	11 Sep	12 Sep	13 Sep	14 Sep	15 Sep	16 Sep	17 Sep	18 Sep	19 Sep	20 Sep
Date of birth expected date	Next 18 Jun	Next 19 Jun	Next 20 Jun	Next 21 Jun	Next 22 Jun	Next 23 Jun	Next 24 Jun	Next 25 Jun	Next 26 Jun	Next 27 Jun
Last menstrual start date	21 Sep	22 Sep	23 Sep	24 Sep	25 Sep	26 Sep	27 Sep	28 Sep	29 Sep	30 Sep
Date of birth expected date	Next 28 Jun	Next 29 Jun	Next 30 Jun	Next 1 Jul	Next 2 Jul	Next 3 Jul	Next 4 Jul	Next 5 Jul	Next 6 Jul	Next 7 Jul

If the last menstrual period started in October

Last menstrual start date	1 Oct	2 Oct	3 Oct	4 Oct	5 Oct	6 Oct	7 Oct	8 Oct	9 Oct	10 Oct
Date of birth expected date	Next 8 Jul	Next 9 Jul	Next 10 Jul	Next 11 Jul	Next 12 Jul	Next 13 Jul	Next 14 Jul	Next 15 Jul	Next 16 Jul	Next 17 Jul
Last menstrual start date	11 Oct	12 Oct	13 Oct	14 Oct	15 Oct	16 Oct	17 Oct	18 Oct	19 Oct	20 Oct
Date of birth expected date	Next 18 Jul	Next 19 Jul	Next 20 Jul	Next 21 Jul	Next 22 Jul	Next 23 Jul	Next 24 Jul	Next 25 Jul	Next 26 Jul	Next 27 Jul
Last menstrual start date	21 Oct	22 Oct	23 Oct	24 Oct	25 Oct	26 Oct	27 Oct	28 Oct	29 Oct	30 Oct
Date of birth expected date	Next 28 Jul	Next 29 Jul	Next 30 Jul	Next 1 Aug	Next 2 Aug	Next 3 Aug	Next 4 Aug	Next 5 Aug	Next 6 Aug	Next 7 Aug
Last menstrual start date	31 Oct
Date of birth expected date	Next 7 Aug

If the last menstrual period started in November

Last menstrual start date	1 Nov	2 Nov	3 Nov	4 Nov	5 Nov	6 Nov	7 Nov	8 Nov	9 Nov	10 Nov
Date of birth expected date	Next 8 Aug	Next 9 Aug	Next 10 Aug	Next 11 Aug	Next 12 Aug	Next 13 Aug	Next 14 Aug	Next 15 Aug	Next 16 Aug	Next 17 Aug
Last menstrual start date	11 Nov	12 Nov	13 Nov	14 Nov	15 Nov	16 Nov	17 Nov	18 Nov	19 Nov	20 Nov
Date of birth expected date	Next 18 Aug	Next 19 Aug	Next 20 Aug	Next 21 Aug	Next 22 Aug	Next 23 Aug	Next 24 Aug	Next 25 Aug	Next 26 Aug	Next 27 Aug
Last menstrual start date	21 Nov	22 Nov	23 Nov	24 Nov	25 Nov	26 Nov	27 Nov	28 Nov	29 Nov	30 Nov
Date of birth expected date	Next 28 Aug	Next 29 Aug	Next 30 Aug	Next 31 Aug	Next 1 Sep	Next 2 Sep	Next 3 Sep	Next 4 Sep	Next 5 Sep	Next 6 Sep

If the last menstrual period started in December

Last menstrual start date	1 Dec	2 Dec	3 Dec	4 Dec	5 Dec	6 Dec	7 Dec	8 Dec	9 Dec	10 Dec
Date of birth expected date	Next 7 Sep	Next 8 Sep	Next 9 Sep	Next 10 Sep	Next 11 Sep	Next 12 Sep	Next 13 Sep	Next 14 Sep	Next 15 Sep	Next 16 Sep
Last menstrual start date	11 Dec	12 Dec	13 Dec	14 Dec	15 Dec	16 Dec	17 Dec	18 Dec	19 Dec	20 Dec
Date of birth expected date	Next 17 Sep	Next 18 Sep	Next 19 Sep	Next 20 Sep	Next 21 Sep	Next 22 Sep	Next 23 Sep	Next 24 Sep	Next 25 Sep	Next 26 Sep
Last menstrual start date	21 Dec	22 Dec	23 Dec	24 Dec	25 Dec	26 Dec	27 Dec	28 Dec	29 Dec	30 Dec
Date of birth expected date	Next 27 Sep	Next 28 Sep	Next 29 Sep	Next 30 Sep	Next 1 Oct	Next 2 Oct	Next 3 Oct	Next 4 Oct	Next 5 Oct	Next 6 Oct
Last menstrual start date	31 Dec
Date of birth expected date	Next 7 Oct