Glossary: Terms Starting with "G"

Gestational age measures how far along a pregnancy is, counted in weeks and days starting from the first day of your last menstrual period (LMP), even though conception happens about two weeks later. Doctors and midwives use gestational age for all clinical milestones, screening schedules, and due date calculations. At your first antenatal appointment, your doctor will ask for your LMP date and may confirm gestational age with an early ultrasound. Understanding your gestational age helps you track which prenatal tests to book and what developmental changes to expect each week.

Related: EDD, fetal age

The gestational sac is the first structure visible on an early pregnancy ultrasound, typically appearing from around 4 to 5 weeks after your last menstrual period. It looks like a small, dark, fluid-filled oval within the lining of the uterus. Seeing a gestational sac confirms the pregnancy is in the uterus, which rules out an ectopic pregnancy. However, a sac alone does not confirm that the embryo is developing normally. Doctors look for the sac to grow at an expected rate and, by 6 weeks, for a yolk sac or embryo to appear inside it before confirming a viable pregnancy.

Related: Yolk sac, embryo

A growth scan is an ultrasound done in the second or third trimester to check whether your baby is growing at a healthy rate. The sonographer measures your baby's head, abdomen, and thigh bone, then plots these on a growth chart. Results are compared to your baby's earlier measurements to track progress over time. Your doctor may order a growth scan if your bump is measuring unusually small or large, if you have gestational diabetes, or if a previous pregnancy had growth problems. Single readings matter less than the overall trend across multiple scans.

Related: EFW, BPD, AC +1 more

The glucose challenge test (GCT) is a quick screening test for gestational diabetes, usually done between 24 and 28 weeks of pregnancy. You drink a sweet glucose solution, and one hour later a blood sample is taken to measure how your body handled the sugar. You do not need to fast beforehand. If your blood sugar reading comes back above the threshold, you will be referred for a longer oral glucose tolerance test to confirm the diagnosis. The GCT itself does not diagnose gestational diabetes. It simply identifies women who need further evaluation so that any blood sugar issues can be managed early for your and your baby's safety.

Related: GDM, OGTT

Group B Streptococcus (GBS) is a type of bacteria that lives harmlessly in the digestive and vaginal tracts of roughly one in four adults. During pregnancy it causes no symptoms, but if passed to a baby during birth it can cause serious newborn infections including sepsis, meningitis, and pneumonia. That is why doctors screen for it late in pregnancy, typically around 35 to 37 weeks, using a vaginal and rectal swab. If you test positive, you will be offered intravenous antibiotics during labor to greatly reduce the risk of passing the bacteria to your baby. Most GBS-positive mothers have healthy babies without any complications.

Related: GBS swab, antibiotics in labor

The GBS swab is a simple, quick test done late in pregnancy, usually between 35 and 37 weeks, to check whether Group B Streptococcus bacteria are present. A swab is gently taken from around the vagina and rectum and sent to a lab for culture. Results are usually available within a few days. The swab is not painful and takes only a moment. A positive result means antibiotics will be recommended through a drip during active labor to protect your baby from infection at birth. Even if you tested negative in a previous pregnancy, you should be retested each time, as your GBS status can change.

Related: Group B strep

Gestational hypertension means your blood pressure rises above 140/90 mmHg after 20 weeks of pregnancy, but without the protein in urine or organ problems seen in preeclampsia. It affects roughly one in ten pregnancies. Although it sounds less serious than preeclampsia, it still puts extra strain on your heart and kidneys, and it can develop into preeclampsia. Your doctor will check your blood pressure at every antenatal visit, and may ask you to monitor it at home. In some cases, medication is needed. Most cases resolve within 12 weeks after delivery, but women who have had it carry a higher risk of high blood pressure later in life.

Related: Preeclampsia, BP monitoring

Gestational diabetes occurs when pregnancy hormones interfere with insulin, causing blood sugar to rise higher than normal. It is typically detected between weeks 24 and 28 through an Oral Glucose Tolerance Test (OGTT). Left unmanaged, it can cause your baby to grow very large, raising the risk of a difficult delivery, low blood sugar in the newborn, and a higher chance of caesarean section. Most women manage it through a carbohydrate-controlled diet and regular walking, though some need insulin injections. Blood sugar usually returns to normal after birth, but having gestational diabetes roughly triples your lifetime risk of developing Type 2 diabetes.

Related: GCT, OGTT