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Medical News Today: 12 weeks pregnant: your pregnancy week by week

During week 12 of your pregnancy, your baby’s organ systems have practically all formed and will continue to grow and start functioning as your pregnancy goes on.1

As with other earlier weeks, week 12 is no exception when it comes to rapid growth and huge developmental strides. In fact, in only 3 weeks, your little bundle of joy has nearly doubled in size!1

This MNT Knowledge Center feature is part of a series of articles on pregnancy. It provides a summary of each stage of pregnancy, what to expect, and insights into how your baby is developing.

You will also see introductions at the end of some sections to any recent developments that have been covered by MNT‘s news stories. Also look out for links to information about related conditions.

Symptoms at 12 weeks pregnant

At this stage of your pregnancy, you may continue to experience physical pregnancy symptoms such as1:

A close-up of a nose.
During week 12 of pregnancy, you may find that you have an increased sense of smell.
  • Dizziness
  • Changes in sex drive
  • Less frequent need to urinate
  • Fatigue
  • Excessive saliva
  • Bloating or gas
  • Heightened sense of smell
  • Increase in vaginal discharge
  • Occasional headaches.

You may or may not be showing, but you can rest assured that your womb is growing. During this time of your pregnancy, your uterus is the size of a large grapefruit.1 Soon, you may feel the need to wear looser fitting clothes or even maternity clothes.

Be aware that pregnancy increases the risk of urinary tract infections from week 6 to week 24, so if your symptoms are not simply from the pregnancy and you suspect an infection, speak with a health care provider about treatment.2

Your hormones at 12 weeks pregnant

Throughout your pregnancy, you will experience variations in certain hormones that contribute to many of the pregnancy symptoms you may experience. Following implantation of the fertilized egg, your body begins to secrete a hormone called human chorionic gonadotrophin (hCG) – the hormone used to detect pregnancy.3 This hormone is also responsible for regulating estrogen and progesterone and contributes to frequent urination.3

Initially produced by the corpus luteum, progesterone rises throughout your pregnancy and continues to do so until the birth of your baby.3 In early pregnancy, progesterone is responsible for increasing uterine blood flow, establishing the placenta and stimulating the growth and nutrient production of the endometrium (lining of the uterus).3

Progesterone also plays a crucial role in fetal development, preventing premature labor and lactation, as well as strengthening the pelvic wall muscles to prepare your body for labor.

In addition to progesterone, the placenta is vital in secreting important hormones during your pregnancy such as3:

  • Human placental lactogen: this hormone is believed to be responsible for mammary gland growth that will be important for lactation following the birth of your baby. Additionally, it plays a role in increasing nutrient levels in your blood, vital to the growth and development of your baby.
  • Corticotrophin-releasing hormone: this hormone is not only responsible for determining how long you will be pregnant, but also for your baby’s growth and development. Later in pregnancy, the rise in both corticotrophin-releasing hormone and cortisol both completes fetal organ development and provides the mother with a surge of cortisol that has been linked with maternal attentiveness, increasing the mother-baby bond.

Another vital hormone in pregnancy is estrogen, which is responsible for fetal organ development, placental growth and function and mammary gland growth.3 Additionally, estrogen is needed for the regulation of other hormones produced during pregnancy.3

Because of the rise in progesterone and estrogen, you may experience some unpleasant pregnancy symptoms such as mood swings and morning sickness.3 Another hormone, relaxin, can cause physical symptoms such as pelvic pain, balance difficulties and constipation due to its role in relaxing maternal muscles, ligaments and joints.3

Genetic testing at 12 weeks pregnant

It is important to speak with your health care provider to determine if genetic testing and certain forms of prenatal screening are right for you. Some prenatal screening tests can begin around 10 weeks gestation.4

From weeks 11-14, an ultrasound may be obtained to evaluate your baby’s nuchal translucency – the amount of fluid under the skin behind the baby’s neck.4 This is important in testing for some medical conditions that may affect your baby such as Down syndrome.4

There are three blood tests offered to women during their pregnancy to screen for a variety of genetic abnormalities including Down syndrome, Trisomy 18, and spina bifida.4

These tests include:4

  • Sequential integrated screen: between weeks 10-13 and again during weeks 15-20, this screening test may be recommended to test the maternal blood for the six fetal proteins found in maternal blood circulation. This test is capable of identifying 92% of Down syndrome pregnancies. The first portion of this test is completed in combination with a nuchal translucency ultrasound (as mentioned above).
  • Serum integrated screen: as in the sequential integrated screen, maternal blood is tested during weeks 10-13 and again during weeks 15-20 for the six fetal proteins found in maternal blood circulation. This test is capable of identifying 88% of Down syndrome pregnancies. This test is typically carried out in cases where a nuchal translucency ultrasound is unavailable.
  • Quad marker screen: obtained during weeks 15-20, the quad marker screen tests maternal blood for four circulating fetal proteins. This test is capable of identifying 79% of Down syndrome pregnancies. Women who did not get the first portion of the serum and sequential tests (as above) are eligible to undergo this testing. This test is also the second test obtained during the sequential and serum screening tests.

Another test offered to women during this time of pregnancy, weeks 10-12, is chorionic villus sampling (CVS).4 CVS is typically offered to women who have other abnormal screening results, are aged 35 years or older, have a prior history of a chromosomal abnormality found in pregnancy or have a family history of certain genetic abnormalities.4

CVS is used to evaluate for genetic abnormalities such as Tay-Sachs disease, cystic fibrosis and certain chromosomal disorders such as Down syndrome.4 This is not a blood test or an ultrasound screening method as discussed above. Instead, during this test, a sample of placental chorionic villi are removed and analyzed.4

Baby’s development at 12 weeks pregnant

Image of a fetus at 12 weeks of pregnancy.
By the time you are 12 weeks pregnant, your baby’s sex organs are visible and his or her heartbeat can be heard.

At 12 weeks pregnant, there are many changes in your baby’s development. Developments that are underway include1,5:

  • Head and neck: the head is half of the size of the body, salivary glands start functioning
  • Heartbeat: the heartbeat can now be heard with an external Doppler
  • Chest: lungs continue to mature, amniotic fluid is inhaled and exhaled
  • Abdomen: spleen and liver remove old red blood cells and produce antibodies
  • Pelvis: sex organs fully visible
  • Limbs: arms proportional with increasingly functional hands, legs remain short
  • Skin: sweat glands are present, body hair is growing, muscles and nervous system are maturing
  • Other changes: the bone marrow is producing white blood cells; the pituitary gland is manufacturing hormones.

Lifestyle changes at 12 weeks pregnant

As with earlier weeks, you will soon find there are many lifestyle modifications that need to be made during pregnancy and even after delivery.

General health

During pregnancy, you will need to take care of yourself and your developing baby. Be sure not to drink alcohol or smoke during pregnancy, and avoid all other toxic substances such as drugs during this time.6 Be sure to discuss all medications you are taking with your health care provider to ensure that you should continue use during your pregnancy.6

To nourish yourself and your baby, make sure you eat a healthy diet and take a good prenatal vitamin.6 Another way to maintain your health during pregnancy is to get regular exercise. Speak with your health care provider about your current or desired exercise regimen to make sure it is safe.


Everyone wants to look their best but using permanent hair color is not recommended during the first 12 weeks of pregnancy. Instead, consider using semi-permanent dye.2


While it is safe to eat fish during pregnancy, it is recommended that you limit your intake to 8-12 oz. of fish and shellfish per week.2,7,8

Some examples of fish that are safe to consume during pregnancy include shrimp, salmon, canned light tuna (note: mercury varies can to can), pollock, cod, catfish and anchovies.2,8 If you plan on eating albacore tuna and tuna steak, it is recommended that you limit consuming this fish to 6 oz. per week.7,8

Avoid eating shark, swordfish, tilefish and king mackerel while pregnant, as they contain high levels of mercury which can be harmful to your baby’s brain and nervous system.7 If eating fish from a non-commercial source – a fish you or your family caught, for example – be sure to check with the local health authorities that the waters in which it was caught are safe.7

Always make sure your food is fully cooked and not raw or undercooked. Also avoid uncooked smoked or pickled fish.7 Additionally, it is important to avoid unpasteurized soft cheese, refrigerated pâté, raw or undercooked meat and poultry, cold cut deli meat, foods containing raw eggs (Caesar dressing, etc) and unpasteurized juice, milk and eggnog.7

Need a boost? Caffeine is OK during pregnancy but should be consumed in moderation. Try to keep your caffeine consumption from all sources at or below 300 milligrams daily.7

Baby’s size at 12 weeks pregnant

Three plums.
At 12 weeks pregnant, your baby is the size of a plum.

Your baby is the size of a plum, measuring approximately 2.5 inches in length.1,5

He or she will now weigh in at around half an ounce.

His or her head is around one half of the length from crown to rump and rests upon its neck rather than its shoulders.

If you have questions regarding your pregnancy, be sure to contact your health care provider.

Call your doctor if you are experiencing symptoms of an ectopic pregnancy or miscarriage such as vaginal bleeding or passage of tissue, leaking vaginal fluid, feeling faint or dizzy, low blood pressure, rectal pressure, shoulder pain and severe pelvic pain or cramping.

Recent developments on pregnancy from MNT news

Stress in pregnancy may raise risk for dental caries in offspring

Stress during pregnancy has been associated with a number of poor health implications for offspring, including low birthweight and increased risk of asthma and allergies. But for the first time, a new study suggests chronic stress in pregnancy may increase a child’s risk for dental caries.

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