First Trimester of Pregnancy
Pregnancy runs 40 weeks, split into three trimesters, and full term means making it to at least 37 weeks. A baby born before that is considered premature, and premature babies can face real challenges with breathing, digestion, and growth, which is a big part of why so much of prenatal care is about buying your baby as much time as safely possible.
Here’s the part that confuses almost everyone at first. Pregnancy is counted from the first day of your last period, not from conception. That means by the time you actually conceive, you’re already considered about two weeks pregnant, even though there was no baby yet. It’s a strange system, but it’s the one your doctor’s office, your due date calculator, and this page all use, so it’s worth just accepting rather than fighting. Our Your Pregnancy, Month by Month guide covers this same stretch broken into monthly chunks if you’d rather read it that way.
How Long Is the First Trimester
The first trimester lasts about 13 weeks, from the first day of your last period through week 13. Some sources stop at week 12. It covers roughly months 1 through 3. You can read about each one on our pages for month 1, month 2, and month 3. Our guide to the trimesters of pregnancy shows how the weeks line up. To see exactly where you are, try the pregnancy weekly calculator or the baby due date calculator.
Fetal Development
This is roughly when fertilization actually happens. Sperm meets egg in the fallopian tube, and that single cell starts dividing almost immediately, combining 46 chromosomes that will determine everything from eye color to blood type. About a day and a half later, the dividing embryo drifts out of the fallopian tube toward the uterus. Every so often it doesn’t make that trip and implants in the tube instead, which is what causes an ectopic pregnancy, a real medical emergency rather than a normal variation.
The embryo has reached the uterus and is looking for a good spot to settle in, then burrows into the uterine lining once it finds one. It’s still barely a sixth of an inch long, but a lot is already underway. The backbone, spinal column, and nervous system are starting to form, and so are the kidneys, liver, and intestines.
This is usually around when a missed period tips you off that something’s different, since the developing embryo is already producing hormones that pause your cycle. At this stage it’s called a blastocyst, a fluid filled cluster of cells attached to the uterine wall. One half becomes the placenta, the support system that will feed your baby for the rest of the pregnancy, and the other half becomes the baby itself. A sheet of cells along its back folds into a tube that will become the spinal cord, with one end already enlarging into what will be the brain.
Your baby is about the size of a raisin now. Small folds low on the sides of the head mark where ears will eventually form, and every major organ system has at least started, even though none of them are close to finished. The placenta is now functioning enough to start passing along nourishment and oxygen through the umbilical cord. The spine is growing faster than everything else at this point, which gives the embryo a brief tail like appearance that disappears as the rest of the body catches up.
Around day 26, a tiny heart starts beating for the first time. Your baby is about a quarter inch long. The kidneys and liver are underway, and the neural tube connecting brain and spinal cord closes up. This is also roughly when a lot of people start feeling early pregnancy symptoms firsthand, nausea, fatigue, sore breasts, and needing the bathroom constantly.
Hands and feet look like little paddles right now, though fingers are just starting to show. The heart has split into left and right chambers and is beating around 150 times a minute, close to double an adult’s resting rate. A mouth, tongue, and the beginnings of eyes with a retina and lens are visible, and your baby is already practicing movement even though you can’t feel it yet.
From here on your baby is officially called a fetus rather than an embryo. Floating in the amniotic sac, it’s about three quarters of an inch long, with arms and legs stretching out and a head that looks oversized simply because the brain is growing so fast. Brain activity can already be measured. Teeth, the palate, and the larynx are starting to form, and the skin is thin enough that you’d be able to see veins right through it.
Constant motion is the theme this week, even though it’ll be a while before you can actually feel any of it. Organs, muscles, and nerves are all in place and starting to switch on. Fingers and toes are shedding their webbed, paddle like shape, and tiny touch pads are forming on the fingertips.
Your baby weighs about a third of an ounce now, with a heart that’s nearly as developed as a newborn’s. A bypass valve keeps most blood away from the still unused lungs, since oxygen is still coming through the placenta. Eyelids have fused shut and will stay that way until around week 27. Wrists, ankles, fingers, and toes are all clearly visible, and external genitals are forming, though it’s still too early for an ultrasound to tell you the sex.
The big internal organs, liver, kidneys, intestines, brain, and lungs, are fully formed and beginning to function, even though your baby is only about 2 inches long and weighs less than half an ounce. The head still takes up close to half the total body length at this stage, and your baby is busy moving and kicking, out of sight for now.
Vocal cords are complete, so your baby can technically cry, just silently, with no air in the lungs yet to make sound. The brain keeps developing at a remarkable pace, and thumb sucking sometimes starts around now. Fingernails and toenails are forming, hair is appearing on the head, and the kidneys are developed enough to start producing urine.
How You May Feel
Every body reacts differently. Some people feel almost nothing, and others feel awful. Common signs include tiredness, nausea, tender breasts, needing the bathroom more often, strong reactions to smells, and mood swings. Mild cramping can happen too. Nausea isn’t limited to mornings, and it can last all day.
For many people the queasiness eases as the second trimester begins. Small, frequent snacks and plenty of fluids help in the meantime. Our page on the signs and symptoms of pregnancy goes into more detail.
Your First Prenatal Visit
Many providers schedule the first visit somewhere between weeks 8 and 12, and some want to see you sooner. Expect a health history, a physical exam, blood and urine tests, and a conversation about your due date. Write your questions down beforehand. It’s easy to forget them once you’re there.
Our prenatal appointment timeline shows what comes next, and the ultrasound page explains what your scans can show. If you haven’t chosen a provider yet, read about picking the right caregiver for your baby’s birth.
Tests in the First Trimester
According to March of Dimes, cell-free DNA screening can be done after week 9, and first trimester screening usually happens at weeks 11 to 13. Chorionic villus sampling, a diagnostic test, is done at weeks 10 to 13 for those who need it. Screening is your choice. Your provider can explain what each test can and can’t tell you.
What tests will my doctor run during the first trimester? →Vitamins and Habits That Matter Now
Start a prenatal vitamin if you haven’t already. The CDC says women who could become pregnant should get 400 micrograms of folic acid every day. The neural tube that becomes the brain and spine forms in the first few weeks, often before you know you’re pregnant.
Ask your provider about alcohol, smoking, and any medicines or supplements you take. No amount of alcohol has been proven safe in pregnancy, so most providers advise skipping it. You don’t need extra calories yet, according to the CDC. Good food and water matter more than eating more. The pregnancy weight gain chart shows what to aim for later.
When to Call Your Provider
Call right away if you have vaginal bleeding, severe belly pain, or a fever of 100.4 degrees or higher. Call if you feel dizzy or faint, too. The same goes for vomiting so much that you can’t keep fluids down for 8 hours or more. These come from the CDC’s list of urgent maternal warning signs.
Sharp pain on one side with bleeding can signal an ectopic pregnancy. So can pain that reaches your shoulder. Both need care right away.
Spotting is common and doesn’t always mean something is wrong, but call anyway. Miscarriage is most common in the first trimester. If it happens, it’s rarely because of anything you did.
Questions Parents Ask
What if I don’t have any symptoms?
That’s normal too. Symptoms vary a lot from one person to the next, and they can change from week to week. Feeling fine doesn’t mean something is wrong.
When can I hear my baby’s heartbeat?
It’s often visible on an ultrasound around week 6. Many providers can hear it with a handheld device around weeks 10 to 12. If it takes a little longer, that’s usually because of dating or your baby’s position.
When does morning sickness go away?
For many people it eases around the start of the second trimester. For some it lasts longer. If you can’t keep food or fluids down, call your provider, since treatment is available.
Do I need to tell my boss right away?
No. Many people wait until after the first trimester. Check your workplace’s leave rules so you know your options when you’re ready.
The first trimester asks a lot of your body before there’s much to show for it. That’s why it can feel so disorienting. Once you’re through it, our Second Trimester guide picks up where this one leaves off. Our nutrition guide is worth revisiting too, since what you need shifts as pregnancy goes on.
Sources
- Fetal Development, The First Trimester – Mayo Clinic
- Fetal Development, Week-by-Week Stages of Pregnancy – Cleveland Clinic
- Prenatal Tests, March of Dimes
- About Folic Acid, CDC
- Urgent Maternal Warning Signs, CDC
Worth knowing. This page is general guidance and can’t replace the advice of your own doctor or midwife. For more to read, browse our pregnancy section.