The World of Pregnancy

How to Calculate Your Due Date?

Even if the possible due date was calculated based on the ultrasound examination or the last menstruation, you shouldn`t have full confidence in it. Only 4% from all babies are born exactly at the calculated date, while 96% of them are born a few days or weeks before or after the due date.

How to Calculate Your Due Date

This can be explain by the fact that, besides the errors which may appear when calculating the possible due date, a normal period of pregnancy may vary from 37 to 42 weeks, and babies born during this period are considered born at term.

After estimating the due date, it`s correct to assume that birth, with the same probability, may happen in a period which starts with 2 weeks before the due date and lasts up to a week after this date.

Due Date based on the Pregnancy Term

Knowing the exact term of the pregnancy (estimated by the ultrasound investigation), the due date can be easily calculated. The duration of a normal pregnancy is averagely of 280 days (starting with the first day of the last menstruation) or 40 weeks. – More details!

Therefore, if you want to calculate the probably due date, you should subtract the actual pregnancy term from 40 weeks and mark this number in the calendar, starting with the current day.

Due Date based on the Last Menstruation

The calculation of the pregnancy term and due date by taking into consideration the last menstruation is based on the assumption that in the case of the average length of the menstrual cycle (the period between 2 menstruations), equal with 28 days, ovulation and conception may be, averagely, in the 14th day from the beginning of the last menstrual cycle.

This calculation method is pretty inexact for the following reasons:

  • Around 20% – 40% from women aren`t able to say the exact date of the last menstruation.
  • The duration of the last menstrual cycle in different women can also be different. For instance for some women, the period between 2 menstruations can be of 21 days, while for others of 42 days.
  • For lots of women, the menstrual cycle is irregular. This means that the moment of ovulation can occur in completely different days of the menstrual cycle (closer to the beginning or the end of the menstrual cycle).

Read more on 15 Things You Need “To-Do” Before You`re Due!

To calculate the possible due date based on the first day of the last menstruation, you have the possibility of using 2 methods:

  • At the first day of the last menstruation, it`s added 7 days, and then it`s counted 9 months after in the calendar, from the obtained date. For example, if the last menstruation started on June 5th, the birth can be expected on March 12th (5 + 7 = 12 June; 12 June + 9 months – 12 March).
  • At the first day of the last menstruation, it`s added 7 days, and then it`s counted from the obtained date, 3 months before in the calendar. For example, if the last menstruation started on June 5th, the birth can be expected on 12 March (5 + 7 = 12 June, 12 June – 3 months = 12 March).

Due Date & Gestational Age Based on Ultrasound

Ultrasound is one of the most exact modern methods of establishing a gestational age and due date.

In the first trimester of pregnancy, it`s possible to determine the pregnancy term based on measuring the diameter of the gestational sac, the fetus`s length or by determining the fetal heart rate.

The pregnancy term in days is: the diameter of the gestational sac + 30.

The gestational age in weeks is: –0.0007 (CRL­) 2+ 0.1584 (CRL) + 5.2876 (+/- 6 days).

The first fetal heart beats are determined, averagely, in the 5th – 6th week of pregnancy.

In the second trimester of pregnancy, to calculate the gestational age may be used the value of the maximum width of fetus`s head (biparietal diameter, BPD).

The pregnancy term in days is: 39.1 + 2.1 (BPD) (+/- 8 days).

In the third trimester of pregnancy, the gestational age is calculated considering the maximum width of the fetus`s head, skull`s circumference, femur length or the presence of the outbreaks of formation of the bone tissues in the large bones of the fetus`s skeleton.

Read more on 8 Signs Showing your Due Date Is Around the Corner!

In around 20% of cases, the due date calculated based on the first day of the last menstrual cycle, is significantly different from the date calculated by ultrasound. In cases in which these 2 dates are different with less than 10 days, it`s considered to be correct the due date calculated based on the first day of the last menstruation. If these 2 dates are different with more than 10 days, it`s considered to be correct the due date calculated by ultrasound.

Establishing the Gestational Age during a Pelvic Examination

Establishing the gestational age depending on the size of the uterus or the pregnant woman`s abdomen is a inaccurate method because first of all, the correct measurement of the uterus`s volume is very difficult, and on the other hand, the size of the abdomen and width of the uterus vary a lot from one woman to another. Currently, this method of calculating the pregnancy term practically isn`t used.

Gestational Age & Due Date Based on the First Fetal Movement Noticed

The date when the woman felt the first fetal movement can only be used for calculating a precise due date.

The observations show that pregnant women feel the first fetal movements for the first time around 19 – 21 week of pregnancy, while women at their second or more pregnancies can feel them at 1 – 2 weeks before (17 – 19 week of pregnancy).

Pregnancy Term & Due Date Based on Intercourse that Led to the Conception

The date of intercourse which led to the conception may be used to calculate the due date only if there`s no doubt that the conception took place immediately after the moment that you are taking into consideration (for example, if this was the only intercourse in the last 3 – 4 months). – Find out more!

If you know when you had your sexual contact that conceived the future baby, count from that date 40 weeks and you`ll know the approximate due date.

Image courtesy of WikiHow.com
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