You are now entering a vital part of your second trimester as you move into the 18th week. Most of the important tests and scans required in the second trimester are performed between the 18th and 22nd weeks. So, consult your doctor regarding the exams and screening necessary for evaluating the growth of your baby.
Baby development in 18th week of pregnancy
Internal organ development
The large intestine moves toward the back wall of the abdomen while different digestive glands begin to form . This is also the week when the uterus moves into its final position along with the fallopian tubes in a female fetus .
Nervous system development
This week is crucial for the development of the nervous system as the tiny nerves are now covered with a protective layer (the myelin sheath), which helps massages to travel faster between nerve cells . The myelin sheath will continue to develop throughout the pregnancy and for one year after your baby is born . The nerves also start forming complex connections while the brain further specializes in sending out signals for the senses of hearing, touch, smell, sight and taste .
Fetal development in twin pregnancy week 18
A thin, transparent membrane separates the two fetuses in a twin pregnancy. According to many researches, twin fetuses can sense each other at this early stage despite their sealed eyelids and lack of vision .
How big is your baby?
Your baby is around 5.6 inches (14.22 cm) long , weighing almost 7 ounces (198.4 gm), about the size of a sweet potato.
Fetal movements at week 18
You may recognize your baby’s movements for the first time this week, if you mistook those little flutters (quickening) earlier for something like butterflies or gas . Most first time mothers feel those most awaited punches and kicks between the 17th and 24th weeks while those with subsequent pregnancies often recognize them earlier .
Your belly and body changes this week
At week 18, your baby bump is likely to be evident enough for attracting pregnancy health tips from all quarter, be it your friends, relatives and even total strangers. You are probably gaining some weight as well. Making healthy food choices is important at this stage as the required weight gain in the second trimester is only 12 lbs to 19 lbs  (even less for overweight women). The major changes in your cardiovascular system in the second trimester are responsible for causing your blood pressure to be lower than normal . In addition to these, the hormone relaxin continues to relax the joints and muscles in your abdomen and pelvic area, leading to aches and pains in the region .
18 weeks pregnant tests and ultrasound
A detailed ultrasound is offered at this stage for assessing the health and development of your baby. Referred to as the fetal morphology scan, it helps your doctor to detect any structural anomalies and any breach in the placental position . Your baby is now large enough to be detected by a transabdominal ultrasound which is used for evaluating and measuring the fetal head, abdomen, spine, femur and the limbs . The gender of your baby can also be detected on the ultrasound image.
Another major change is that your doctor can now listen to the fetal heartbeat with a regular stethoscope , so a fetal Doppler is no longer necessary.
Alpha-Fetoprotein Test (AFP) or Maternal Serum Alpha-Fetoprotein Screening (MSAFP) might be recommended if you have not already had this test in the 16th-17th week. It measures the alpha-fetoprotein levels in your blood to assess the chances of your baby having certain genetic disorders. An abnormal AFP result calls for the triple screen test .
Signs and symptoms of pregnancy week 18
- Lightheadedness or dizziness
- Frequent urination 
- Stretch marks
- Increased appetite 
- Indigestion and heartburn
- Varicose veins 
- Gas and bloating
- Leg cramps 
- Round ligament pain 
- Tailbone pain
- Back and joints hurt when you walk, sit or lie down
- Nosebleeds 
- Bleeding gums (due to the pregnancy hormones and increased blood flow)
- Linea nigra (a dark line down the middle of your abdomen)
- Appearance of dark patches on the face and certain other skin areas
- Itchy skin rashes (PUPPP) 
- Shortness of breath 
- Edema or swelling of the arms, legs, hands and feet (due to fluid retention)
- Itchiness, especially around the belly, back, thighs and legs
- Runny or stuffy nose (pregnancy rhinitis) 
- Milky white discharge (leukorrhea) 
- Trouble sleeping (insomnia) and weird pregnancy dreams (your overactive brain in pregnancy is believed to be responsible for this; however, scientists are yet to find the reason behind this behavior of the brain) 
- Anxiousness and mood swings
When to call the doctor?
- Stuffy nose accompanied by symptoms like sore throat, coughing sneezing, headache, fever and vomiting (might indicate flu or some other similar infectious condition) 
- Thin, watery clear or straw colored discharge (might indicate leaking amniotic fluid)
- Vaginal itchiness with lower back pain, burning sensation during urination or dark yellowish or greenish foul smelling discharge (might indicate a kidney infection or yeast infection) 
- Severe abdominal pain or persistent cramping 
- Constant headaches with extreme dizziness and blurred vision (might indicate high blood pressure) 
- Extreme fatigue accompanied by excessive thirst, nausea and vision changes (might indicate gestational diabetes) 
- Severe diarrhea that lasts for over 24 hours 
Tips for a healthy pregnancy and baby
- Including lots of calcium and vitamin D rich foods (milk, cheese, almonds, leafy green vegetables and fruits like apple) in your daily diet as it is a necessary nutrient for the fetal bone ossification and teeth development 
- Avoiding sudden movements like standing up too quickly from a sitting position, as this might lead to dizziness or fainting by further lowering the already low blood pressure levels 
- Opting for a larger shoe size if you have swollen feet, as wearing your regular shoes might lead to further problems by constricting the blood vessels 
- Following a proper dental care routine (brushing and flossing) to prevent the symptoms of swollen and bleeding gums from leading to any periodontal diseases (take care not to injure your already sensitive gums further while brushing) 
- Drinking plenty of water to stay hydrated and improve symptoms like leg cramps and constipation
- Consulting your doctor before taking any over-the-counter medicines, such as Benadryl and Tylenol, for managing the common symptoms 
- Drinking a cup of chamomile or peppermint tea occasionally for severe nausea and heartburn (consult your doctor regarding the safety of herbal tea in your pregnancy) 
- Try to lie on your side (preferably left side) while resting as lying on your back causes the uterus disrupt the blood flow to your heart by putting pressure on a major vein)
- Joining a prenatal yoga class or following a light workout routine that includes pelvic floor exercises to strengthen the muscles and minimize pain and discomfort 
- http://www.babycenter.com.au/a536339/your-pelvic-floor-in-pregnancy [/ref]