13 Weeks Pregnant: Symptoms and Baby Size

At 13 weeks pregnant you are about 3 months along, on the border between the first and second trimesters, with about 27 weeks to go until your estimated due date.

Week 13 is a turning point on paper and often in how you feel. The Office on Women’s Health (OWH) says you might notice symptoms like nausea and fatigue going away in the second trimester, though bodies do not follow a calendar.

This guide collects what major medical sources say about your baby’s development and size, symptoms, the screening window that closes soon, sleep, and when to call. Where sources disagree, we print both.

Key Takeaways

Week 13 is the last week of the first trimester by the ACOG definition UPMC uses, and the first week of the second by the Office on Women’s Health.
You are about 3 months along, with about 27 weeks to go.
Mayo Clinic says bones start to harden around week 13. UPMC puts the fetus at about three inches.
The nuchal translucency ultrasound is done between the 11th and 14th week, so ask now if you want it.
Bleeding, severe belly pain, a temperature above 100.4 F or severe vomiting are reasons to call right away.

Table of Contents

13 Weeks Pregnant at a Glance

Quick Answer

At 13 weeks pregnant you are about 3 months along, with about 27 weeks to go. It is the last week of the first trimester or the first week of the second, depending on the definition. Mayo Clinic says bones start to harden around week 13, and UPMC puts the fetus at about three inches.

13 weeks pregnant	What to know
Trimester	First or second, depending on the source (see the next table).
Month	About 3 months, counting 4 weeks and a few days per month. Months are a convention, not a medical unit.
Weeks to go	About 27 (40 minus 13, counted from the last menstrual period).
Baby’s size	About three inches by week 13 (UPMC).
Common symptoms	Nausea, tiredness, constipation, tender breasts, mood swings.
This week’s care focus	First trimester screening, if you want it. The nuchal translucency test is done between the 11th and 14th week (MedlinePlus).

13 weeks pregnant: quick facts.

Source	First trimester	Where week 13 falls
American College of Obstetricians and Gynecologists (ACOG) definition, as reported by UPMC	Ends once you have passed 13 weeks and 6 days	First trimester (last week)
Office on Women’s Health (OWH)	Week 1 to week 12	Second trimester (week 13 to week 28)
Mayo Clinic fetal development guide	Its second trimester page opens with week 13	Second trimester

Is week 13 the first or second trimester? It depends on the source.

How the weeks are counted. OWH says pregnancy lasts about 40 weeks, counting from the first day of your last normal period. Cleveland Clinic notes that this gestational age runs about two weeks ahead of conception, which is why Mayo Clinic labels week 13 as 11 weeks after conception. UPMC says the first ultrasound, around weeks 11 to 13, confirms the due date, so your provider may adjust your week count.

13 weeks pregnant in months. You have completed about three calendar months. Cleveland Clinic’s guide, which uses four-week months, places weeks 13 through 16 in month 4.

Your Baby’s Development at 13 Weeks

No source can say what your baby is doing on a given day. These references describe what is typical around now.

Source and timing	What it says is happening
Mayo Clinic, week 13	Bones start to harden, especially in the skull and the long bones of the arms and legs. The skin is thin and see-through.
Cleveland Clinic, week 13	Vocal cords form, and the large head begins to grow in proportion to the body.
Cleveland Clinic, week 12	All the organs, limbs, bones and muscles are present and will keep developing.
OWH, at 12 weeks	Nerves and muscles begin to work together. The eyelids close and will not open again until the 28th week.
UPMC, weeks 10 to 13	By the end of the first trimester, the fetal organs start to function.

Fetal development around week 13, by source.

Can you feel movement yet? Cleveland Clinic says that by week 11 the fetus is opening and closing its fists and mouth, but it is too soon to feel any kicks. Its guide says most people begin to feel kicks around week 19.

Boy or girl? OWH says the external sex organs show by about 12 weeks and that an ultrasound in the second trimester or later might be able to find out the sex. UPMC notes that the noninvasive prenatal test (NIPT), a blood test, can also tell the sex. Planning to share it? See our gender reveal ideas.

How Big Is Your Baby at 13 Weeks?

UPMC says the fetus grows from about one inch to about three inches between weeks 10 and 13. The other primary sources we read print figures for the weeks on either side.

Source	Week	Length	Weight
Cleveland Clinic	12	About 2.5 to 3 inches, “about the size of a plum”	Not given
Office on Women’s Health	12	About 3 inches	Almost an ounce
UPMC	10 to 13	Grows from about one to three inches	Not given
Mayo Clinic	14	Almost 3 1/2 inches (87 mm), crown to rump	About 1 1/2 ounces (45 g)

Baby size near 13 weeks. Sources differ, so each is printed.

Mayo Clinic measures crown to rump, from the top of the head to the bottom of the tailbone, so the legs are not included. Not every source states its method. These are averages. The measurement that matters is the one your provider takes on ultrasound.

13 Weeks Pregnant Symptoms

Symptom	What the sources say	What may help
Nausea	OWH lists an upset stomach, with or without throwing up, as a first trimester change and says you might notice nausea going away in the second trimester.	Smaller meals more often (UPMC). A few crackers before bed (MedlinePlus).
Tiredness	UPMC says progesterone rises rapidly in the first trimester, making you tired.	An afternoon nap and short walks (UPMC). An earlier bedtime (OWH).
Constipation	UPMC ties it to slower digestion and says the iron in prenatal vitamins can add to it.	More water and more fiber (UPMC).
Tender breasts	UPMC says the breasts are growing and preparing for breastfeeding.	A comfortable, well-fitting bra.
Mood swings	UPMC attributes them to hormonal changes.	Talking to a friend or a therapist, meditating, yoga or a walk (UPMC).

Common symptoms at 13 weeks pregnant.

OWH also lists frequent urination, headache, heartburn and food cravings, and says most of these discomforts will go away as pregnancy progresses. If yours are not fading, tell your provider: OWH’s wording is that most women, not all, find the second trimester easier.

A gentle word on miscarriage risk. Cleveland Clinic’s wording is that, since the most critical development has taken place, the chance of miscarriage drops considerably after 12 weeks. It does not drop to zero, and no article can tell you how your own pregnancy is going. UPMC says bleeding or spotting can be normal in the first trimester but can also be a sign of a miscarriage or an ectopic pregnancy, so report it.

Baby on the way? Get ahead on sleep.

Our free two-minute quiz builds a gentle, personalized sleep plan you can use from the first weeks at home.

Take the Free Sleep Quiz

Your Body and Belly at 13 Weeks

OWH says that in the second trimester your abdomen expands as the baby continues to grow. None of the sources we read gives a week by which you should be showing, and bodies vary, so a flat belly or a small bump tells you little on its own. Our pregnant belly by month guide shows the usual progression.

Sharing the news. When to tell people is a personal choice, not a medical rule. If you are ready, browse our pregnancy announcement ideas.

Prenatal Care, Tests and Appointments Around Week 13

UPMC says you will see your doctor or midwife about once a month in the first trimester, with the second visit usually around weeks 11 to 13.

Test	Timing	What it is	Source
Nuchal translucency ultrasound	Between the 11th and 14th week	Measures the nuchal fold at the back of the baby’s neck to screen for the chance of Down syndrome and some other conditions	MedlinePlus
First trimester screen and first ultrasound	Around weeks 11 to 13	A blood test plus an ultrasound that measures the baby’s size and the fluid at the back of the neck, and confirms the due date	UPMC
NIPT	Ask your provider	Uses a sample of your blood to analyze the baby’s DNA, assess the risk of genetic problems and tell the sex	UPMC

Tests around 13 weeks, with the timing each source gives.

These are screening tests, not diagnoses. MedlinePlus explains that an abnormal nuchal translucency result is followed by more testing, such as amniocentesis or NIPT. Screening is your choice. If you want the scan and have not had it, call this week, because the window ends at the 14th week.

What to Do This Week: A Simple Checklist

Ask about screening: the timing matters this week (see the table above).
Keep taking your prenatal vitamin: UPMC advises a daily prenatal vitamin with folic acid.
Move most days: the Centers for Disease Control and Prevention (CDC) recommends at least 150 minutes a week of moderate-intensity aerobic activity during pregnancy, for example 30 minutes on 5 days. Brisk walking counts.
Check your routine with your doctor: the CDC says to ask whether or how to adjust your activity, and to try to avoid activities that require lying flat on your back after the first trimester.
Eat small and often: and keep drinking water.
Save your call list: keep your provider’s daytime and after-hours numbers handy.

Sleep at 13 Weeks Pregnant

MedlinePlus says you may sleep well during the first trimester and may need more sleep than usual, and that sleeping well tends to get harder later in pregnancy. If you are still wiped out by early evening, an earlier bedtime (OWH’s suggestion) is a reasonable response.

Sleep position. None of the sources in this guide sets a position rule for week 13 specifically. MedlinePlus’s general advice for pregnancy is to try sleeping on your side with your knees bent, which it says keeps the baby from putting pressure on the large vein that carries blood back from your legs. It adds that many providers tell pregnant women to sleep on their left side. Ask your provider what they recommend for you.

Use pillows: under your belly or between your legs (MedlinePlus). See how to use a pregnancy pillow.
Keep a schedule: go to bed and wake up at the same time each day.
Watch the evening: MedlinePlus advises limiting soda, coffee and tea, and avoiding a lot of fluids or a big meal within a few hours of bed.

MedlinePlus is direct about sleep aids: do not take any, including over-the-counter medicines and herbal products, without talking to your provider. Our guide to melatonin during pregnancy covers the questions to ask.

For Partners: How to Help at 13 Weeks

Come to the appointment: the visit around weeks 11 to 13 often includes the first ultrasound (UPMC).
Make food easy: nausea can strike at any time of day (UPMC). Keep simple snacks around and take over cooking that smells strong to her.
Protect rest: pick up a chore so there is room for a nap or an earlier bedtime.
Learn the warning signs: read the list below. For a small treat, see gifts for pregnant women.

Looking Ahead: What Comes Next

Week 14: the second trimester begins under the ACOG definition UPMC uses, and the nuchal translucency window ends. Mayo Clinic says the baby’s sex likely becomes clearer.
Week 16: Cleveland Clinic lists about 5 inches and about 4 ounces, and says the ears are developed enough to hear you talk.
Weeks 19 to 20: Cleveland Clinic says most people begin to feel kicks around week 19. Mayo Clinic says you might feel movement around week 20.

For the bigger picture, see our first trimester guide. Next in this series: 22 weeks pregnant and 27 weeks pregnant, then the third trimester.

Related Betteroo Guides

When to Call Your Provider

Call any time something worries you. UPMC lists these first trimester warning signs:

Severe nausea and vomiting, which can make it hard to keep down fluids.
Vaginal bleeding or spotting.
Cramping. UPMC says this can be normal but requires a prenatal check-up.
Green or yellow vaginal discharge, or discharge with a bad odor.
Pain when urinating.
A temperature above 100.4 F.

UPMC adds that if your symptoms are serious, you should go to the emergency department. Seek medical care immediately for any of the CDC’s urgent maternal warning signs:

A headache that will not go away or gets worse over time.
Dizziness or fainting, or changes in your vision.
Trouble breathing, chest pain or a fast-beating heart.
Being unable to drink for more than 8 hours or eat for more than 24 hours.
Severe belly pain that does not go away.
Vaginal bleeding that is more than spotting, or fluid leaking from the vagina.
Extreme swelling of your hands or face, or severe swelling, redness or pain in a leg or arm.
Thoughts about harming yourself or your baby.

If you cannot reach your provider quickly, go to the nearest emergency room or call 911, and say that you are pregnant. This article is general education, not medical advice.

Frequently Asked Questions

Is 13 weeks pregnant the second trimester?

It depends on the definition. UPMC, using the American College of Obstetricians and Gynecologists definition, says the first trimester ends once you have passed 13 weeks and 6 days, so week 13 is its last week. The Office on Women’s Health counts the second trimester as week 13 to week 28.

How many months is 13 weeks pregnant?

About 3 months. Thirteen weeks is 91 days, roughly three calendar months counted from the first day of your last period. Guides that use four-week months, such as Cleveland Clinic’s, place weeks 13 through 16 in month 4. Providers track pregnancy in weeks.

How big is my baby at 13 weeks?

UPMC says the fetus grows from about one to three inches between weeks 10 and 13. The Office on Women’s Health lists about 3 inches and almost an ounce at 12 weeks, and Mayo Clinic lists almost 3 1/2 inches from crown to rump at week 14.

Can you feel the baby move at 13 weeks?

Usually not yet. Cleveland Clinic says the fetus is opening and closing its fists by week 11, but that it is too soon to feel kicks. Its guide says most people begin to feel kicks around week 19, and Mayo Clinic says you might feel movement around week 20.

What symptoms are common in week 13?

Nausea, tiredness, tender breasts, constipation and mood swings are on the first trimester lists from the Office on Women’s Health and UPMC. The Office on Women’s Health says you might notice nausea and fatigue going away in the second trimester. Severe vomiting, bleeding or a temperature above 100.4 F are reasons to call.

Can you find out the sex at 13 weeks?

Sometimes, by blood test. UPMC says the noninvasive prenatal test (NIPT) analyzes your baby’s DNA from a sample of your blood and can tell the sex. The Office on Women’s Health says an ultrasound in the second trimester or later might be able to show it.

Calmer days, better nights.

Betteroo builds a personalized sleep plan for your baby. Take the free two-minute quiz and have it ready before the newborn weeks.