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Pregnancy due dates are often estimated by doctors and midwives based on the first day of a woman’s last menstrual period (LMP). The obstetric estimate is then used to plan for prenatal care, delivery, and post-natal care.

When women enter prenatal care in the US, their first visit is usually scheduled for around 8 weeks of pregnancy. The doctor or midwife then calculates the estimated due date by adding 40 weeks to the LMP. In reality, a woman’s LMP may not be pinpoint accurate, so some of those women will deliver earlier and some later than expected.

Data from the National Center for Health Statistics indicates that the standard deviation of all pregnancy lengths is 3 weeks. This means that 68% of women will deliver within 3 weeks of their due date, and 95% will deliver within 6 weeks of their due date.

Because of this, some doctors and midwives may prefer to use the first day of the last menstrual period (LMP) to calculate an estimated due date, and then calculate a ‘due-date range’ instead of a single due date.

For example, a doctor or midwife may say that a woman’s estimated due date is January 20th but her ‘due-date range’ is January 5th to February 2nd.

This gives the doctor or midwife some flexibility in scheduling prenatal visits and allows more time for the baby to arrive before induction or cesarean section would be necessary.

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Variation of Pregnancy Length

Many factors can cause variations in the length of pregnancy. These include ovulation time, fertilization time, implantation time, duration of embryonic and fetal development, gestation length (the number of days from conception to birth), how long it takes for a baby to be born after full-term (40 weeks), etc.

Some of these factors are more predictable than others. For example, it is fairly easy to determine the day of ovulation, but it is not always possible to know exactly when a fertilized egg will implant in the uterus.

Since many women do not know exactly when they ovulated or conceived, and because ovulation can vary greatly from cycle to cycle, LMP-based due dates are often inaccurate.

The American College of Obstetricians and Gynecologists (ACOG) acknowledges that there is a wide range of normal for the length of pregnancy and offers the following suggestions for due date accuracy:

“The average length of gestation varies from 40 weeks to 42 weeks, with a standard deviation of 2 weeks. Thus, 68% of all live births occur between 38 and 42 weeks. However, 95% occur within 1 week either side of the due date.”

What Factors Can Cause Variations in Pregnancy Length?

Factors that affect the length of pregnancy include:

  1. ovulation time
  2. fertilization time
  3. implantation time
  4. duration of embryonic and fetal development
  5. gestation length (the number of days from conception to birth)
  6. how long does it take for a baby to be born after full-term (40 weeks)

Here are some examples of how these factors can affect pregnancy length:

1. Ovulation Time

The length of a woman’s menstrual cycle is highly variable and can be influenced by stress, exercise, travel, illness, or even the consumption of alcohol or caffeine. It is difficult to predict exactly when ovulation will occur.

In addition, it is not possible to predict the baby’s due date using a woman’s last menstrual period alone.

To determine the due date, doctors take into account the first day of a woman’s last menstrual period (LMP) and count back three months from that date. The result is known as an estimated due date (EDD). This method is called Naegele’s Rule.

Unfortunately, there are many cases where the EDD differs from the actual birth date by several weeks or even months. In other words, this method is not always accurate. Doctors do not always use this method to determine a baby’s due date. They often use ultrasound results to get a more accurate estimate of when the baby will be born.

2. Fertilization Time

The sperm can survive in the female reproductive tract for up to 5 days after intercourse. This means that a woman’s estimated due date could be off by up to 5 days depending on when she ovulated and when her partner ejaculated.

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3. Implantation Time

The fertilized egg must travel down the fallopian tube and implant in the uterus for pregnancy to occur. It takes about 6 days for the fertilized egg to travel from the fallopian tube to the uterus, so a woman’s estimated due date could be off by up to 6 days depending on when she ovulated and when her partner ejaculated.

4. Duration of Embryonic and Fetal Development

The length of time that it takes for an embryo to develop into a fetus can vary greatly depending on how many cells it has, how many cells divide each day, etc.

For example, if an embryo has 16 cells at 8 weeks of pregnancy, then it will have 32 cells at 10 weeks of pregnancy, 64 cells at 12 weeks of pregnancy, 128 cells at 14 weeks of pregnancy, 256 cells at 16 weeks of pregnancy, 512 cells at 18 weeks of pregnancy, 1,024 cells at 20 weeks of pregnancy, and so on.

This means that the length of time from conception to birth will vary depending on how many cells the embryo has at a given time.

5. Gestational Length (the Number of Days from Conception to Birth)

The number of days from conception to birth can vary greatly depending on when the fertilized egg implants in the uterus and how many cells it has at a given time. For example:

Make Pregnancy Easy

There is a wide range of normal for the length of pregnancy. This means that due dates are often inaccurate because they are based on LMP instead of ovulation or fertilization time. In addition, ovulation can vary greatly from cycle to cycle, which makes LMP-based due dates even less accurate.

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