An expectant mother holds her hands over her belly. Her hands are shaped into a heart.
Your baby will undergo the most dramatic changes in the first trimester. (For Spectrum Health Beat)

Maybe you took a home pregnancy test for confirmation, or maybe you had blood drawn. Either way, you’ve just found out you’re pregnant.

It’s an exciting time, with lots of amazing activity happening inside your body.

We know that conception needs to first involve three steps: ovulation, fertilization and implantation. Let’s take a closer look at these processes, and at the first trimester of pregnancy.


Ovulation occurs when the egg is released from the ovary.

It’s a myth that all women ovulate on the 14th day of their cycle. Some women ovulate earlier, some later. If you are trying to get pregnant, an ovulation kit is a good way to find out the timing of your ovulation.

Using a thermometer to check your temperature first thing in the morning can also help, although it’s important to keep in mind that your temperature rises after you’ve ovulated. If you want to chart your cycles, is a helpful site.


This happens in the fallopian tubes, when the egg if penetrated by the sperm.

The cells begin to divide right after fertilization and travel to the uterus over about five days, at which point the cells become a blastocyst.

A few days later—about eight to nine days after fertilization—the blastocyst attaches to the lining of the uterus. This is known as implantation.


A third of women will have implantation spotting about eight to 12 days after conception. This could be one of the first signs of pregnancy, although other notable signs include breast tenderness and nausea.

The amniotic sac is also forming at this point.

This marks the beginning of the embryonic stage, which lasts until week eight.

Embryonic highlights

At week five the baby is growing and developing within the lining of the uterus.

There are three layers of cells: inner, middle and outer. The inner layer will become the lungs, digestive organs and bladder. The middle layer will be the circulatory system (heart and blood vessels) and the bones and muscles. The outer layer will be the nervous system, including the brain and skin.

Later, the placenta will take over the feeding of baby, but at this point the baby’s “food” comes from the yolk sac the baby is attached to.

So, what exactly is happening from week five to week eight?

  • The placenta is forming. The placenta is truly amazing. It’s an organ that feeds baby and gets rid of baby’s waste. One end is connected to the inside of the uterus, with the other connects to baby by the umbilical cord. Since your developing baby won’t use lungs to breathe until birth, the placenta serves to deliver oxygen to baby inside the womb.
  • Your baby is developing quickly. The spinal cord and brain are forming and the limbs are looking like little paddles at this point. The ears, nose and eyes develop. Even the eyelids are forming. The genitals are also developing, although it’s too early to know if baby is a boy or girl.
  • Your baby’s heart is beating. The heartbeat can be heard using an ultrasound at about six weeks. This is more common using a transvaginal ultrasound. If it’s an ultrasound on the belly, it may be seven to eight weeks before you can see and hear the heart beating.

Fetal stage

By the end of week eight, your baby’s major body systems and organs have started developing. It’s at this point that your baby is called a fetus. Over the next several weeks, and until the end of the first trimester, these changes occur:

  • Nails begin to form on the finger and toes
  • Muscles and bones continue to grow
  • The skin is transparent and thin
  • The intestinal track forms
  • Teeth buds develop

Here’s a fascinating fact: If your baby’s growth rate in the first trimester continued throughout the pregnancy, that baby would be 15 feet tall just after birth! That would be a big baby.

Luckily, that extraordinary growth rate remains in the first trimester. It’s a period of amazing changes for your baby and your body!