With each week of pregnancy, your baby-to-be is developing in leaps and bounds. In the early stages, they are called a zygote. They then progress to an embryo, fetus, and finally, baby.
You might hear your doctor talk about different phases of pregnancy with specific medical terms like embryo and zygote. These describe your baby’s stages of development.
Fertilization is a process that usually happens within a few hours of ovulation. It’s that critical point in reproduction when the sperm meets the newly released egg. At this meeting, 23 male and 23 female chromosomes mix together to create a single cell embryo called a zygote.
In human pregnancies, a baby-to-be isn’t considered a fetus until the 9th week after conception, or week 11 after your last menstrual period (LMP).
The embryonic period is all about the formation of important systems of the body. Think of it as your baby’s basic foundation and framework.
The fetal period, on the other hand, is more about growth and development so your baby can survive in the outside world.
Weeks 1 and 2: Preparation
You actually aren’t pregnant during the first two weeks (on average) of your cycle. Instead, the body is preparing to release the egg. Take note of when your last period started so you can give this information to your doctor. The LMP will help your doctor date your pregnancy and determine your due date.
Week 3: Ovulation
This week begins with ovulation, the release of an egg into the woman’s fallopian tubes. If the sperm is ready and waiting, there’s a chance the egg will become fertilized and turn into a zygote.
Week 4: Implantation
After fertilization, the zygote continues to divide and morph into a blastocyst. It continues its journey down the fallopian tubes to the uterus. It takes about three days to reach this destination, where it will hopefully implant into your uterine lining.
If implantation takes place, your body will start to secrete human chorionic gonadotrophin (hCG), the hormone that’s detected by home pregnancy tests.
Week 5: Embryonic period starts
Week 5 is important because it starts the embryonic period, which is when the bulk of your baby’s systems will be forming. The embryo is in three layers at this point. It’s only the size of the tip of a pen.
- The top layer is the ectoderm. This is what will eventually turn into your baby’s skin, nervous system, eyes, inner ears, and connective tissue.
- The middle layer is the mesoderm. It’s responsible for your baby’s bones, muscles, kidneys, and reproductive system.
- The last layer is the endoderm. It’s where your baby’s lungs, intestines, and bladder will later develop.
Your baby doesn’t look like the one you’ll bring home from the hospital yet, but they’re gaining some very basic facial features, plus arm and leg buds. The embryo is now the size of a pomegranate seed and has a primitive tube of cardiac cells that emits electrical impulses and pumps blood.
Baby’s brain and head are further developing in week 7. Those buds of arms and legs have turned into paddles. Your baby is still as tiny as a pencil eraser, but they already have little nostrils. The lenses of their eyes are beginning to form.
Your baby’s eyelids and ears are forming so they’ll be able to see and hear you. Their upper lip and nose are also starting to take shape.
Baby’s arms can now bend at the elbow. Their toes are forming, too. Their eyelids and ears are getting more refined.
Week 10: Embryonic period ends
Your baby started as a tiny speck and is still less than 2 inches long from crown to rump. Still, your little one is starting to look like a tiny newborn. Many of their body’s systems are in place.
This is the last week of the embryonic period.
Congratulations, you’ve graduated from having an embryo to a fetus. From week 11 onward, your baby will continue to develop and grow until the end of your pregnancy. Here’s more of what they’re up to.
Late first trimester
Your baby’s development is still in high gear for the rest of the first trimester. They’ve even started to grow fingernails. Their face has taken on more human characteristics. By the end of week 12, your baby will be 2 1/2 inches from crown to rump, and weigh around 1/2 ounce.
Week 13 marks the start of the second trimester. During this stage, your fetus is looking and operating more like a real baby.
Early on, their sex organs are developing, their bones are getting stronger, and fat is starting to accumulate on their body. Midway through, their hair becomes visible, and they can suck and swallow. They can start to hear your voice, too.
Your baby will grow during this time from 3 1/2 inches from crown to rump, to 9 inches. Their weight will go from 1 1/2 ounces to 2 pounds.
Starting at week 27, you’re in the third trimester. In the first half of this stage, your fetus starts to open their eyes, practices breathing in amniotic fluid, and becomes covered in vernix caseosa.
Toward the end, they are gaining weight more quickly, making lots of big movements, and starting to crowd themself in the amniotic sac.
Your fetus starts the third trimester at 10 inches from crown to rump, and grows to 18 to 20 inches. Their weight starts at 2 1/4 pounds and goes up to 6 1/2 pounds. The length and weight of babies at delivery varies greatly.
Early pregnancy can be difficult on your mind and emotions. Researchers estimate that between 10 to 25 percent of all clinically recognized pregnancies end in miscarriage (pregnancy loss before 20 weeks).
Many of these miscarriages happen in the earliest stages of development, even before you’ve missed your period. The rest usually happen before week 13.
Reasons for miscarriage might include:
- chromosomal abnormalities
- underlying medical conditions
- hormone issues
- woman’s age at conception
- failed implantation
- lifestyle choices (e.g., smoking, drinking, or poor nutrition)
Contact your doctor if you’re pregnant and experience vaginal bleeding (with or without clots), cramping, or a loss of pregnancy symptoms. Some of these symptoms may be normal, but it’s a good idea to double check.
When you get a positive pregnancy test, call your doctor to set up your first prenatal appointment.
At this meeting, you’ll typically go over your medical history, discuss your due date, and have a physical exam. You’ll also get an order for lab work to check for existing infections, blood type, hemoglobin, and your immunity against different infections.
Important questions to ask at your first appointment include:
- When is my due date? (Try to remember when your last menstrual period was. Your doctor may use an ultrasound to date your pregnancy.)
- What types of vitamins do you recommend I take?
- Are my current medications and supplements OK to continue during pregnancy?
- Are my current exercises or work activities OK to continue during pregnancy?
- Are there any foods or lifestyle choices I should avoid or modify?
- Is my pregnancy considered high-risk for any reason?
- How much weight should I gain?
- What should I do if I feel like something is wrong? (Many providers have after-hours on-call staff ready to answer your questions.)
Most doctors see patients about every four weeks during the first and second trimesters of pregnancy. These appointments give you an excellent opportunity to ask questions, monitor your baby’s health, and catch potential maternal health issues before they become bigger problems.
Your baby hits a lot of milestones and markers before their delivery date. Each stage is important in the overall pregnancy picture.
As your baby continues to develop, try to focus your efforts on taking care of yourself, keeping up with your prenatal appointments, and connecting with the life growing inside you.
- Changes during pregnancy. (n.d.). https://www.acog.org/womens-health/infographics/changes-during-pregnancy
- FAQs: How your fetus grows during pregnancy. (2022). https://www.acog.org/womens-health/faqs/how-your-fetus-grows-during-pregnancy
- Pascual ZN & Langaker MD. (2022). Physiology, pregnancy.