Skip to Content

View Additional Section Content

Pregnancy Milestones: What to Know about the Stages of Prenatal Development

Pregnancy marks an exciting time in parents’ lives. Whether you’re expecting your first child or your fifth, it’s always fun to be able to decorate a nursery, pick out potential names and share the good news with friends and family. But pregnancy also marks an important period of time in your baby’s development. Prenatal development is key to a child’s health and wellness after birth.

We spoke with Bethany Perry, MD, an obstetrician-gynecologist at Jefferson Health - Abington, about some of the major milestones in prenatal development and how you can keep both you and your baby healthy throughout pregnancy.

Early Stages

The germinal stage of pregnancy starts at the time of conception, when the sperm and the egg combine to form a zygote. During the germinal stage, the zygote begins to divide in order to implant into the uterine wall. Once implantation is complete, the embryonic stage begins.

It’s important to note that prenatal development is measured in weeks from the mother’s last menstrual period, as well as in trimesters. The embryonic stage lasts about eight weeks.

“During these eight weeks, the organ systems will begin to form and develop and the embryo will look like a little tadpole, growing to about 1 to 1.5 inches by the end of this period,” says Dr. Perry.

In the latter half of the embryonic stage, tooth buds, arms and legs can be seen. This is also the time that most women discover that they are pregnant, at around four weeks after conception.

Fetal Stage

The fetal stage of prenatal development lasts from week nine until birth. During this stage, the fetus will continue to develop organs, including the brain and spinal cord. Between weeks nine and 12, the fetus will begin to make movements with their arms and legs. It’s also during this stage that sex organs will begin to differentiate. Gender can usually be determined around 18-20 weeks by ultrasound or earlier if you require genetic testing.

At week 12, the second trimester of the pregnancy begins. At this stage, the fetus is about 6 inches long. Dr. Perry states, “The second trimester is when your baby will start to grow hair, open their eyelids, grow eyebrows and eyelashes, develop fingerprints and have a huge blossom in brain development.” The baby can start to develop reflexes to stimuli, and you may be able to feel moving and kicking between 18 to 20 weeks into your pregnancy.

The third trimester starts at week 27 and lasts until birth. Your baby will be around 15 inches long at the beginning of the third trimester. It’s at the end of this period that the baby will develop their lungs and digestive system to full maturity.

The fetus will also continue to put on weight in preparation for leaving the womb. Your baby is considered full-term at 37 weeks, but most women will go into labor between 39 and 41 weeks. Dr. Perry advises, “Only about 5 percent of women give birth on their actual due date.”

Your doctor will always walk you through the important stages and milestones of your pregnancy, and if you have any questions or concerns during pregnancy you should contact them right away.

Find a Physician
Search Our Directory


Schedule a