How to calculate the duration of pregnancy

Apr 27, 2025 Source: Cainiu Health

Dr. Zhang Lu

Introduction

Under normal circumstances, pregnancy refers to gestation. The gestation period is calculated from the first day of the last menstrual period, and the entire pregnancy lasts approximately 280 days, or 40 weeks. Women who are planning a pregnancy should also track their menstrual cycles during the preconception period and can use methods such as basal body temperature measurement and ovulation test strips to determine their ovulation period, thereby increasing the chances of conception.

Generally speaking, pregnancy refers to gestation. The gestation period is usually calculated from the first day of the last menstrual period, and the entire pregnancy lasts about 280 days, or 40 weeks. A detailed explanation is as follows:

Most women find it difficult to determine the exact time of fertilization, whereas the first day of the last menstrual period is a clear reference point. Therefore, this time is generally used as the starting point of pregnancy. Women with regular menstrual cycles typically ovulate about 14 days before the next menstrual period, and fertilization usually occurs within 24 hours after ovulation. Calculating the gestational period from the first day of the last menstrual period allows for a more standardized division of the pregnancy stages. The pregnancy period is generally divided into three stages: early pregnancy (1-13 weeks + 6 days), mid-pregnancy (14-27 weeks + 6 days), and late pregnancy (28-40 weeks). This classification helps medical staff monitor and assess the condition of the pregnant woman and the fetus. For example, during early pregnancy, attention can be focused on embryo implantation and organ formation; during mid-pregnancy, screening tests such as Down syndrome screening can be conducted to detect fetal chromosomal abnormalities and structural malformations; and in late pregnancy, the focus shifts to fetal maturity and preparation for delivery.

Women who are planning a pregnancy should also record their menstrual cycles during the preconception period and can determine the ovulation period through methods such as basal body temperature measurement and ovulation test strips to increase the chances of conception. At the same time, maintaining a healthy lifestyle, such as balanced nutrition and moderate exercise, will help prepare for conception and a healthy pregnancy.

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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.