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Fetal Development.


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Within 24 hours after fertilization, the egg that will become your baby rapidly divides into many cells. By the eighth week of pregnancy, the embryo develops into a fetus. There are about 40 weeks to a typical pregnancy. These weeks are divided into three trimesters.

What are the three stages of fetal development called?
Fetal development is an orderly and intricate process. It begins before you even know you’re pregnant and ends with the birth of your baby. Between conception and delivery, there are many detailed steps that have to occur.

There are three stages of fetal development: germinal, embryonic, and fetal. Most people don’t talk about their pregnancy in these terms, but it can be helpful to know.

Germinal stage
The germinal stage is the shortest stage of fetal development. It begins at conception, when a sperm and egg join in your fallopian tube. The sperm fertilizes the egg and creates a zygote. The zygote begins its journey down to your uterus over the course of about one week. During this journey, the zygote divides many times, eventually creating two separate structures. One structure eventually becomes the embryo (and later, the fetus), and the other becomes the placenta. Cell division continues at a rapid pace. Eventually, the zygote turns into a blastocyst. The blastocyst arrives at your uterus and implants into your uterine lining. If implantation is successful, your body immediately begins producing hormones to support a pregnancy. This also stops your menstrual period.

Embryonic stage
The embryonic stage lasts from about the third week of pregnancy until the eighth week of pregnancy. The blastocyst begins to take on distinct human characteristics. It’s now called an embryo. Structures and organs like the neural tube (which later becomes the brain and spinal cord), head, eyes, mouth, and limbs form. The embryo’s heart begins to develop and pulse around the sixth week. Buds that will become arms and legs also form around the sixth week. By the end of the eighth week, most of the embryo’s organs and systems have taken shape. For a lot of people, this is the point in pregnancy where morning sickness begins.

Fetal stage
The fetal stage of development begins around the ninth week and lasts until birth. This is when the embryo officially turns into a fetus. The fetus gets its assigned sex around nine weeks of pregnancy, although your healthcare provider can’t detect it on ultrasound yet. The fetus’s major organs and body systems continue to grow and mature. Things like fingernails, eyelashes, and hair also grow. The fetus is able to move its limbs, although you may not feel it until 20 weeks of pregnancy. The majority of growth—in both weight and length—happens in the fetal stage.

When does a pregnancy start?
The start of pregnancy is actually the first day of your last menstrual period (LMP). This is the gestational age of the fetus. It’s about two weeks ahead of when conception actually occurs. Though it may seem strange, the date of the first day of your last period will be an important date when determining your due date. Your healthcare provider will ask you about this date and will use it to figure out how far along you are in your pregnancy.

How does conception work?
Each month, your body goes through a reproductive cycle that can end in one of two ways. You’ll either have a menstrual period or become pregnant. This cycle is continuously happening during your reproductive years—from starting your period to menopause around age 50.

In a cycle that ends with pregnancy, there are several steps. First, a group of eggs (called oocytes) gets ready to leave your ovary for ovulation (the release of the egg). The eggs develop in small, fluid-filled cysts called follicles. Think of these follicles as small containers for each immature egg. Out of this group of eggs, one will become mature and continue through the cycle.

The mature follicle now opens and releases the egg from your ovary. This is ovulation.

After ovulation, the opened follicle develops into a structure called the corpus luteum. This releases the hormones progesterone and estrogen. Progesterone helps prepare your uterine lining for pregnancy. If you don’t become pregnant during a cycle, this lining is what your body sheds during your period. If sperm fertilizes the egg, conception occurs, and the fertilized egg begins its journey to your uterus, where it will implant.

How long is a pregnancy?
Traditionally, we think of pregnancy as a nine-month process. However, this isn’t always the case. A full-term pregnancy is 40 weeks, or 280 days. Depending on what months you’re pregnant during (some are shorter and some longer) and what week you deliver, you could be pregnant for either nine months or 10 months. This is completely normal and healthy.