• The first trimester of pregnancy lasts from weeks 1 through 12.
• During the first trimester, the fetus will grow to about the size of a lime.
• Common symptoms during your first trimester include tender breasts, acne, nausea, and constipation.

As your baby grows and your body starts to transform during pregnancy, every week brings new changes. Sherry Ross, MD, an OB-GYN with her own practice, says the first 12 weeks - aka first trimester - are often accompanied by symptoms like:

• Nausea and vomiting
• Extreme fatigue
• Frequent urge to urinate
• Breast tenderness

Being prepared for your pregnancy and knowing what to expect can help you feel more comfortable and take better care of yourself. So, here's a week-by-week breakdown of what to expect during your first trimester of pregnancy.

Important: The actual start of your pregnancy is hard to pin down, since the exact date of conception is hard to determine. Physicians calculate how many weeks pregnant you are by starting with your last normal menstrual period and using that as the first to second week of your pregnancy.

Pregnancy symptoms typically start three to four weeks after what physicians mark as your first day of pregnancy.

3 to 4 weeks pregnant

• How big is the baby: A tiny ball of cells, roughly the size of a poppy seed.
• What's developing in your womb: The embryo attaches itself to the lining of your uterus, a process known as implantation.
• What are common symptoms: You may experience symptoms like spotting or cramping around the time of implantation, which can be mistaken for a period.
• What to look out for: Taking a home pregnancy test towards the end of week four can help you check whether you're pregnant. "Delay pregnancy testing as close to your expected period as possible," says Ross. If you're unsure of the result or want further confirmation, you can visit your healthcare provider for a blood test to confirm your pregnancy.

5 to 6 weeks pregnant

• How big is the baby: Less than a quarter-inch long, about the size of an apple seed.
• What's developing in your womb: Beginnings of the baby's eyes, ears, arms, legs, brain, spinal cord, and heart. The placenta and umbilical cord start to develop and the baby's heart begins beating.
• What are common symptoms: Some of the first tell-tale symptoms of pregnancy often kick in around this time and you may experience fatigue, breast tenderness, frequent urination, cravings, nausea, and vomiting - although not just in the morning.
• What to look out for: Substances that could pose a risk to your baby and are best avoided include cat litter, alcohol, drugs, certain medications (like medications that treat high blood pressure and seizures), and foods like raw eggs, raw or processed meats, fish high in mercury (like tuna), fresh or unpasteurized cheeses, unpasteurized dairy products, and juices.

7 to 8 weeks pregnant

• How big is the baby: Less than a half-inch long, or about the size of a blueberry.
• What's developing in your womb: The face, head, and brain continue to grow rapidly and the hands, feet, genitalia, lungs, and digestive system start to develop.
• What are common symptoms: Your clothes may start to feel tighter as your womb grows. You might also have symptoms like nausea, extreme fatigue, tender or enlarged breasts, and frequent urination. You may also face other common issues around this time, like constipation, feeling faint or warmer than usual.
• What to look out for: If you haven't already, you may have your first prenatal visit around week eight. Your healthcare provider will probably ask for a detailed medical history and conduct a series of tests. You may have your first ultrasound around this time. "Always bring your questions and concerns to the attention of your healthcare provider," says Ross.

9 to 10 weeks pregnant

• How big is the baby: A little over an inch-long, or about the size of a prune.
• What's developing in your womb: The beginnings of the baby's facial features, fingers, toes, elbows, teeth, reproductive organs, and hormones.
• What are common symptoms: Your breasts may have gotten bigger and you may have a lot of mood swings thanks to hormonal changes. Another symptom you can expect around this time is increased vaginal discharge, which should be white or clear, thin, and odorless.
• What to look out for: Around this time, your healthcare provider will give you the option to screen for genetic abnormalities that can help predict conditions like Down's syndrome.

11 to 12 weeks pregnant

• How big is the baby: Now referred to as a "fetus," the baby is around 2.5 inches long and weighs about half an ounce, roughly the size of a lime.
• What's developing in your womb: Fingernails, toenails, and vocal cords have started to form. From now until week 20, the baby will grow rapidly. As the muscles start to develop, the baby will make a lot of small, jerky movements that you won't be able to feel yet, although you'll be able to see them on an ultrasound.
• What are common symptoms: The combination of pregnancy hormones and increased blood volume will give you radiant and glowing skin, often referred to as the "pregnancy glow." Some people also get acne or headaches due to the surge of hormones. You may also have symptoms like constipation, gas, stomach pain, or cramps.
• What to look out for: If your ultrasound shows a heartbeat by week 12, chances are you have a healthy pregnancy, says Ross. Usually, after eight weeks your risk of a miscarriage begins to go down considerably. Ross says the first trimester of pregnancy has the highest risk of miscarriage with over 85% of miscarriages happening during this period.

Insider's takeaway

To ensure you and your baby remain healthy, Ross says to take your prenatal vitamins regularly, eat a well-balanced diet, get plenty of rest, and check with your healthcare provider before taking any prescription medication.

Pay attention to how you are feeling and be aware of any potential warning signs of abnormal pregnancy like vaginal bleeding, pelvic pain, and uterine cramping, says Ross.