An Overview of Fetal Development

Looking at ultrasound

Fetal development is the term used when talking about the procession of events that starts when sperm and egg meet and unfolds in an orderly progression to form a baby. For something that "simply happens" once a woman gets pregnant, it is amazingly complex—and a lot of fun to think about.

Counting Pregnancy

It is important to know that pregnancy lasts about 40 weeks from the first day of your last period to the completion of the pregnancy, labor, and birth. Your practitioner will use the weeks to help figure out where you are in pregnancy and what should be happening at what point. These weeks are also broken into trimesters:

  • First Trimester (Weeks 1 to 13)
  • Second Trimester (Weeks 14 to 27)
  • Third Trimester (Weeks 28+)

These methods of counting are more official and practical. They will be used in all of your medical charts and decisions. You may still have people ask you how many months along you are in your pregnancy. This is fun to count, but pregnancy doesn’t quite add up to exactly nine months.

Stages of Fetal Development

When you talk to people who study genetics and fetal development, they look at pregnancy in a very different way. The three stages that they look at are:

  • The Germinal Stage (Weeks 2 to 4)
  • The Embryonic Stage (Weeks 5 to 9)
  • The Fetal Stage (Weeks 10+)

The Germinal Stage (Weeks 2 to 4)

This earliest stage of development is one of the least known for most people. This begins as the egg and sperm meet in the outer third of one of the fallopian tubes. Once the two become one, a zygote results and it continues its journey toward the uterus. At this point, the body is not recognizing the pregnancy has occurred. It can take seven or more days to travel the length of the tube and deposit the fertilized egg in the waiting uterus.

The uterus has built up a lining in anticipation of a fertilized egg. This entire journey has seen cell division going from a single cell. In the beginning, all of the cells are identical. It isn’t until the cells reach the eight-cell stage that they start differentiating the type of cells they will be. The inner cells begin to form what will be the embryo. There are three layers:

  • Ectoderm (skin and nervous system)
  • Endoderm (respiratory and digestive system)
  • Mesoderm (muscle and skeletal system)

The outer cells (trophectoderm) develop in the placenta. Once the blastocyst begins to implant into the uterus, human chorionic gonadotropin (hCG) is released, which allows the mother’s body to detect the pregnancy. Eventually, this hCG will be detectable in enough quantity that a pregnancy test will turn positive. Once implantation has occurred, it will signal the body to alter the body’s chemistry to prevent the menstrual cycle from arriving again until the pregnancy is over.

The missing menstrual cycle is typically what will trigger most women to take a pregnancy test.

The Embryonic Stage (Weeks 5 to 9)

The cells are now considered to be an embryo. Though now very distinctly human in origin, the embryo still does not look like what most of us picture when we think of a baby. The embryonic period is very critical because every organ system is formed.

One system that gets a lot of discussion in this critical time period is the neural tube (what eventually becomes the spinal cord, nervous system, and brain). This begins to form 22 days after conception, about 36 days from the first day of your last period. Spina bifida and anencephaly are two types of neural tube defects that can occur, particularly when there is not enough folic acid in the body. This is one of the biggest reasons for the push for all women of reproductive age to take prenatal vitamins or at least folic acid. It can be very difficult to pinpoint pregnancy this early, particularly since about 50 percent of all pregnancies are unplanned.

The baby’s heart is also rapidly forming. It starts as a single blood vessel that begins to pulse around the fifth week of pregnancy. It is still too early to hear the this even using Doppler technology. This won’t happen until around week 10, although a transvaginal ultrasound can pick up tiny pixels fluttering as the blood vessel beats with activity starting around weeks six to seven. A baby’s heartbeat is much more rapid than an adult’s, but it starts slowly, picks up rapidly (heading toward 180 beats per minute), and then settles into the 120 to 160 range for the rest of the pregnancy in the fetal stage.

The body is also forming. You will see images with holes or dark spots that become nostrils, eyes, the mouth, and ears. You will also see arm and leg buds, rapidly changing to include joints (elbows and knees). You will see distinct finger and toe rays a bit later in this period.

While the decision as to whether or not the baby is female or male was genetically decided at conception, every baby looks the same at this point externally (though the external sex organs are present, you cannot tell a clitoris from a penis).

This embryonic stage is only five weeks long. By the end of this time period, the embryo will weigh about the same as a paper clip and be about an inch long, and yet possess nearly every organ system and structure needed for external life.

The Fetal Stage (Weeks 10+)

The term fetus is one that many people have heard. This is the technical name for the baby in the fetal stage and is Latin for "offspring" or "newly delivered fruit." The fetal stage is seemingly less exciting. While everything is present, there are many nuances and much fine-tuning to prepare the fetus for life outside the womb.

Between 12 and 14 weeks, you can just begin to differentiate the boys from the girls via external sex organs, though—even using ultrasound—it is hard to be accurate with sex determination at this phase. That is best done between weeks 18 and 22 during a fetal anatomy scan. A baby girl will be born with every egg she will ever have in her life inside her ovaries, while a baby boy does not have sperm in his testes.

There are things that you may not have thought about in terms of growing like fingerprints, eye lashes, hair, and teeth. Even the permanent teeth have begun forming during this part of pregnancy. The body is covered with a fine hair called lanugo and there is a coating on the skin called vernix caseosa.

Around the third trimester, or 28 weeks from the last period, the nervous system begins to respond more like that of a newborn baby. You can notice that your baby seems to have regular periods of rest and activity, just like a newborn. Your baby will even practice breathing the amniotic fluid, which is partially comprised of fetal urine.

Certainly the fetus will go from being a one-gram, one-inch being to weighing roughly seven pounds and being about twenty inches long, but the fetal stage is about than growing in weight and height. The organ systems require a lot of nuanced changes. For example, the brain of the baby will grow in size and shape, but it isn’t until the very last weeks that the folds of the brain deepen and the weight gain in the brain is significant. (This is one of the many reasons why the end of pregnancy is crucial for the health and wellness of your baby.)

Complications With Fetal Development

There are things that can alter the course of healthy fetal development at the genetic level, as well as physical issues that can interfere. Sometimes these problems will halt the process all together and the baby will stop growing and pregnancy will end. This is more likely in the germinal stage, when the mother does not even know she is pregnant, or in the embryonic stage, where she may or may not know she is pregnant.

A genetic or physical problem may also cause an anomaly that does not effect the viability of the baby, but is still obvious. This might be something like Down syndrome (genetic) or club foot (physical).

There are many things that could go wrong, but thankfully they don’t do so often. The vast majority of babies see their development go unimpeded to birth.

Genetic Screening

If a family has a history of genetic issues or if the mother is over age thirty-five, it is reasonable for genetic screening to be offered prior to or during pregnancy. Prior to pregnancy, a family may be screened for genetic diseases like Tay-Sachs disease, sickle cell disease, and others. Once pregnant, the focus of testing shifts to actually screening the particular pregnancy and baby for anomalies.

It is important to understand the difference between screening your baby and actual genetic testing. Screening would indicate the risk you or your baby has of having a particularly disease. This is usually compared to the average risk for someone of your background and age. So a positive screen would indicate that your testing reveals that you or your baby is a greater risk than average for your age and background.

Once you have a positive screen, either by blood work or by ultrasound, you should be offered genetic testing. Since these tests have a small but real risk of pregnancy loss, it is not recommended for everyone to use these tests. The two most common ones are chorionic villus sampling (CVS) and amniocentesis. You will work with your practitioner, your genetic counselor, and others to explore the process.

Health of the Pregnancy

The health of the pregnancy will depend on the health of the mother and the partner to a certain degree. This includes the health of both in the months leading up to the pregnancy. This is one of the reasons that preconceptional health visits and reproductive life planning are important, as recommended by the Centers for Disease Control and Prevention (CDC).

Prenatal care begins once the pregnancy is confirmed and will continue through the completion of the pregnancy. This includes preventive care, screening, and treatment of complications and potential complications as they arise.

Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  • American College of Obstetricians and Gynecologists (ACOG). Prenatal Development: How Your Baby Grows During Pregnancy. FAQ156, June 2015.
  • Ethical issues in genetic testing. ACOG Committee Opinion No. 410. American College of Obstetricians and Gynecologists. Obstet Gynecol 2008;111:1495–502.
  • Moos MK. From Concept to Practice: Reflections on the Preconception Health Agenda. Journal of Women’s Health; March 2010;19(3):561-7.
  • Preconception Health and Health Care: The Clinical Content of Preconception Care. American Journal of Obstetrics and Gynecology; December 2008; Vol 199, No. 6, p. S257-S396-Supplement B.
  • Robbins, CL, Zapata, LB, Farr, SL. Core State Preconception Health Indicators — Pregnancy Risk Assessment Monitoring System and Behavioral Risk Factor Surveillance System, 2009. MMWR 2014;63(No. SS-3).

By Robin Elise Weiss, PhD, MPH
Robin Elise Weiss, PhD, MPH is a professor, author, childbirth and postpartum educator, certified doula, and lactation counselor.