How Long is Pregnancy? How to Calculate the Term Right?

How long is pregnancy? The truth is that it is impossible to say exactly how many weeks a pregnancy lasts. Doctors assure that the period of the baby’s stay in the mother’s stomach from the moment of conception to birth is 40 weeks. In reality, the baby can choose the date of his birth himself. The doctor calculates the approximate date of birth at the first visit of a future mother. And after receiving the results of an ultrasound examination, this date is most often adjusted.

So, how long is pregnancy? How to calculate it yourself? About all the nuances of such an important date, read in the article.

How Long is a Pregnancy?

The calculations of the gynecologist do not always correspond to the real date. Acceptable error is two weeks. Pregnant women often get angry and confused because of this inaccuracy. And almost all future women in labor wish to independently calculate the date of birth.

The main obstacle to accurate calculations is that it is usually difficult to say exactly when the conception happened. Most often, pregnancy occurs during ovulation, when a mature egg leaves the follicle and is ready to accept the sperm.

Special tests have been developed to determine the date of ovulation. For this purpose, an ultrasound examination is also done.

Depending on the quality and speed of movement of the sperm, fertilization of the egg may not occur immediately after sexual intercourse, but a few days later.

After the sperm penetrates the woman’s reproductive cell, it begins to move to the uterus. And having reached it, it is attached to the wall. Only after this, you can begin the countdown of 40 weeks of pregnancy.


If women in your family have a tendency to miscarriage or have given birth prematurely, it is possible that you will repeat this scheme as well.

It often happens that a woman programs herself on some date of birth, and the baby is born exactly on this day.

Common myths that girls often appear a little earlier, and boys later, are not confirmed.

Duration of Pregnancy: Essential Terms

When registering in the antenatal clinic, the first thing an expectant mother hears is the question from the doctor: what is the date of the last menstruation? From this day, the gestational age is counted in the medical record. But between the date of the last menstruation and the date of conception, usually, at least two weeks pass. So when does the countdown start?

Doctors distinguish between:

  • Obstetric (gestational) gestational age – it is calculated from the day of the last menstruation; the average duration is 40 weeks (280 days).
  • Embryonic (fertilization) gestational age – from fertilization to childbirth; the average duration is 38 weeks (266 days).

Obstetricians sometimes use their count of weeks and months: in a month they have exactly 28 days, i.e. exactly 4 weeks.

The result is that in 9 regular calendar months there are 10 obstetric months.

Embryonic Term: How to Calculate the Date of Conception

It is not always possible to determine the exact date of conception. All women are different, and each pregnancy is unique. At the same time, the date of the last menstruation is specific and reliable information.

Therefore, the answer to the question: how many obstetric weeks does the pregnancy last? – is 40 weeks (280 days). Based on this information, the doctors can predict the estimated date of birth.

The fetal term is more difficult to calculate. As a rule, doctors consider the menstrual cycle of 28 days for most women. Ovulation occurs on the 14th day (the egg leaves the ovary). Fertilization occurs during the same period.


  • First day of the last menstruation + 14 days = ovulation (conception)
  • That means the actual gestational age is 40 – 2 = 38 weeks, or 280 – 14 = 266 days

We add 266 days to the date of the proposed ovulation and we get the approximate date of birth.

But these are the rules, and life is full of exceptions. The woman’s health status, her emotional mood, sperm activity of her partner, etc. make their corrections.

Doctors consider the following gestation periods to be the norm:

  • Pregnancy in weeks – 38-40
  • Pregnancy in days – 266-280
  • Months – 9 calendar months (or 10 obstetric months -each month is exactly 4 weeks)

A blood test for the level of human chorionic gonadotropin (hCG) and ultrasound can help to clarify the term of pregnancy in the first trimester. Ultrasound determines the size of the uterus and the stages of development of the fetus. It is important to remember: an ultrasound can objectively determine the duration of pregnancy only until the 16th week.

Premature and Long-Term Pregnancy

Each particular pregnancy is influenced by many factors and circumstances that can lead to both preterm birth and a long-term one.

Preterm Pregnancy

Pregnancy is considered preterm, and childbirth is premature if the baby was born before the 37th week. For a woman, this situation is less dangerous than for a child. His body is not yet ready for an independent life.

Long-Term Pregnancy

This is less common than premature birth. Such a diagnosis is made after the 42nd week of pregnancy.

Too late labor is dangerous for both: the mother and the baby since the placenta is no longer functioning normally and doesn’t supply the baby with everything necessary. But even if the placenta copes, the fetus may already be too large. This is fraught with birth injuries for the child and ruptures and severe bleeding for the woman.

How Long is Pregnancy? What Affects its Length?

The following may affect the duration of pregnancy and cause premature or late birth:

  • Excessive physical activity – in the later stages even ordinary household chores or the weight of food packages are dangerous
  • Emotional state – severe stress can start the birth process ahead of time; and the opposite happens as well – if a woman is not psychologically ready for childbirth or is simply afraid of it, her fears are able to slow down the start of labor
  • Medical preparations – there are a number of drugs that can affect the natural course of pregnancy, lead to both a miscarriage or premature birth of the baby, and to a delay in the onset of labor
  • General health status of a woman both before pregnancy and during gestation
  • Congenital (genetic) predisposition – if a woman’s mother or grandmother had preterm children, then the risk of untimely birth could be inherited
  • Multiple pregnancy is many times more complex and unique process than carrying one baby. Such
  • Childbirth more often occurs prematurely; the norm is considered when twins are born at 36-37 weeks, and triplets at 33-35 weeks.