As a subject matter expert in human development and obstetrics, I can provide a detailed explanation on the duration of the fetal stage. The fetal stage is a critical period in human development, following the embryonic stage, and it is characterized by significant growth and maturation of the fetus.

The embryonic period, which is marked by the major organ systems beginning to form, typically lasts for about 8 weeks post-conception. This period is crucial for the initial development of the baby, as it sets the foundation for further growth.

Following the embryonic period is the fetal period, which is generally considered to start from the 9th week of gestation and continues until birth. During this phase, the fetus undergoes substantial growth and development. The organs that began forming during the embryonic stage continue to mature, and the fetus starts to take on a more human-like appearance.

In obstetrics, the duration of pregnancy is often calculated from the first day of the Last Menstrual Period (LMP), which is a standard method for estimating the due date. This method adds approximately two weeks to the gestational age calculated from conception, as it includes the time before implantation when the woman is not yet pregnant.

The fetal period can be further divided into three trimesters for ease of description and medical monitoring:


1. First Trimester (Weeks 9-12): The fetus continues to grow rapidly, and by the end of this period, it is about 3 inches long and weighs nearly an ounce.


2. Second Trimester (Weeks 13-26): This is a period of significant development, where the fetus grows to about 14 inches in length and weighs about 1.5 pounds by the end of this trimester.


3. Third Trimester (Weeks 27-40): The fetus gains the most weight during this time, preparing for life outside the womb. By the end of this period, the baby is considered full term and is typically around 20 inches long and weighs between 5.5 to 8.8 pounds.

It's important to note that the exact duration of the fetal stage can vary from pregnancy to pregnancy. Some babies are born prematurely, while others may be carried to post-term, beyond the typical 40 weeks of gestation.

In conclusion, while the embryonic period lasts approximately 8 weeks, the fetal period extends from the 9th week until birth, which is roughly 30 weeks when measured from the date of conception. However, when calculated from the LMP, which is the standard obstetric practice, the total duration of the fetal stage, including the embryonic period, is about 40 weeks.

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The embryonic period (A) lasts 8 weeks and the fetal period (B) from the 9th week to the birth, i.e., 30 weeks. In obstetrics the pregnancy weeks (PW) are normally reckoned from the date of the Last Menstrual Period (LMP).read more >>