Zygote, Embryo, Fetus – Learn the Differences

There’s nothing like a human developing inside your own body. Pregnancy is a fascinating and thrilling experience where your body can experience so many unusual changes and unforeseen events that can spring up.

You will hear your doctor speak about various stages of pregnancy using specific clinical terminologies like zygote, embryo, and fetus. These explain the stages of development of your baby. 

Here, you can learn more about the difference between these words, what your child is up to while in your womb, and what you should expect along the way. Your baby-to-be grows leaps and boundaries with each week of pregnancy.

The Importance of the Zygote Phase Prenatal Development
Image Source: Verywell Family


The mucus in the cervix becomes more fluid and more elastic at ovulation, enabling sperm to quickly penetrate the uterus. Sperm can move into the uterus from the vagina, via the cervix, and to the funnel-shaped edge of the fallopian tube, the usual point of fertilization, in 5 minutes.

Fertilization is facilitated by the cells lining the fallopian tube. Fertilization is a process that typically occurs within a couple of hours of ovulation. When the sperm meets the freshly released egg, it’s the vital moment of reproduction.

Fertilization occurs when a sperm penetrates the egg. The fertilized egg (zygote) is pushed through the tube into the uterus by tiny hairlike cilia lining the fallopian tube. When the zygote travels down the fallopian tube to the uterus, the cells of the zygote divide repeatedly.

The zygote continues to divide and develop into a blastocyst after fertilization. It continues its route to the fallopian tubes to the uterus. Reaching its destination takes about three days, where it will hopefully be implanted into your uterine lining.


Your body will start to secrete human chorionic gonadotrophin (hCG), the hormone that is identified by home pregnancy tests if implantation actually occurs. The embryo under the lining of the uterus on one side, which forms within the amniotic sac, is the next stage of development.

The development of most internal organs and external body structures characterizes this process called the embryonic period or embryo stage. Consider it as the solid framework and foundation of your child.

Week 5 is crucial because the embryonic phase begins, which is when the bulk of the systems of your baby will develop. At this stage, the embryo is in three layers. It is just the size of a pen’s tip.

The top layer is the ectoderm. The middle layer is the mesoderm, and the last layer is the endoderm. See the table below to know what these layers will eventually turn into.

Nervous systemmusclesintestines
inner earsReproductive system 
connective tissue  

A baby-to-be is not considered a fetus in human pregnancies until the ninth week after conception or week 10 after the last menstrual period (LMP).


You progressed from having an embryo to a fetus after week 10. The disparity between the embryo and the fetus is dependent on gestational age. Your baby will start developing at the fetal stage and grow until the duration of your pregnancy. 

The systems that have already been shaped continue to evolve or grow at this stage. During pregnancy, below are indicators.

  • At 12 weeks of pregnancy, the whole uterus is filled by the fetus.
  • Around 14 weeks, it is possible to identify sex.
  • At about 16 to 20 weeks, usually, a pregnant woman can feel the fetus moving. Normally, women who have been pregnant before experience movements about 2 weeks earlier than women who are first pregnant.
  • By 24 weeks, the fetus is likely to survive outside the uterus.

Until close the time of delivery, the lungs proceed to mature. Throughout pregnancy and the first year of life after birth, the brain accumulates new cells. It extends tiny hairlike projections (villi) into the wall of the uterus as the placenta grows.

In a complex tree-like arrangement, the projections branch and branch. By 18 to 20 weeks, the placenta is fully developed but continues to evolve during pregnancy, you can see it via ultrasound. It weighs around 1 pound upon delivery.

Cygote, Embryo and Fetus: What Are the Differences?
Image Source: Babygest


The union of the sperm cell and the egg cell includes a zygote. Often known as a fertilized ovum, in the days following fertilization, the zygote starts as a single cell but divides rapidly. 

Eventually, the zygote becomes an embryo after this two-week cycle of cell division. The egg becomes a fetus if all goes well.