How to calculate the duration of pregnancy

May 08, 2025 Source: Cainiu Health

Dr. Zhang Lu

Introduction

Under normal circumstances, there are several methods to calculate the duration of pregnancy, including the last menstrual period method, ovulation estimation method, ultrasound examination method, physical examination method, and fetal movement timing method. Pregnant women are advised to ensure adequate sleep, which helps with physical recovery and fetal development. Additionally, during early pregnancy the fetus is not yet stable, so strenuous activities and excessive fatigue should be avoided to prevent risks such as miscarriage.

Generally, there are several methods for calculating the duration of pregnancy, including the last menstrual period method, ovulation estimation method, ultrasound examination method, physical examination method, and fetal movement dating method. A detailed explanation is as follows:

1. Last Menstrual Period Method

The duration of pregnancy is calculated from the first day of the last menstrual period to the current date. For example, if the last menstrual period started on January 1st and the current date is February 10th, the pregnancy duration is 41 days.

2. Ovulation Estimation Method

Women with irregular menstrual cycles can estimate the ovulation period to determine gestational age. Normally, a woman's ovulation period usually occurs about 14 days before the next menstrual period. Adding 14 days to the ovulation period provides a rough estimate of the pregnancy duration.

3. Ultrasound Examination Method

In early pregnancy, doctors may use an ultrasound to measure the size of the fetus and thereby estimate the gestational age. For example, measurements such as the biparietal diameter of the fetal head, crown-rump length, and femur length can be used with specific formulas to calculate the gestational age in weeks.

4. Physical Examination Method

Doctors estimate gestational age by palpating the uterus. The size and position of the uterus change with gestational age; for example, around 12 weeks of pregnancy, the uterus enlarges beyond the pelvis. Based on experience, doctors can roughly estimate the gestational age in weeks.

5. Fetal Movement Dating Method

Pregnant women can estimate gestational age based on the date they first feel fetal movements. Typically, fetal movements begin around 18–20 weeks of pregnancy. Some women may perceive fetal movements slightly earlier, before 18 weeks. If the first perception of fetal movement occurs at 20 weeks, subtracting 20 weeks gives an approximate gestational age.

Pregnant women are advised to ensure adequate sleep to support bodily recovery and fetal development. Additionally, as the fetus is not yet stable during early pregnancy, strenuous activities and excessive fatigue should be avoided to prevent risks such as miscarriage.

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Can a woman get pregnant without breaking the hymen?

The hymen is generally referred to as the vaginal membrane. Under normal circumstances, pregnancy is still possible even if the vaginal membrane remains intact, although the probability is relatively low. Whether or not pregnancy occurs primarily depends on whether sperm meets and fertilizes an egg, rather than on the integrity of the vaginal membrane. It's important to note that if you do not plan to become pregnant, protective measures should be taken during any intimate contact, regardless of whether the vaginal membrane is intact.

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What are the reasons for questioning the effectiveness of fertility awareness-based (calendar) methods of contraception?

A woman's ovulation time can be affected by emotions, stress, environment, and other factors, potentially causing it to occur earlier or later than expected. This can render the originally calculated safe period inaccurate and increase the risk of unintended pregnancy. For women with irregular menstrual cycles, it is difficult to accurately predict ovulation and the safe period. Relying on this method for contraception significantly increases the likelihood of failure due to miscalculation.