Glossary: Terms Starting with "O"

Ovulation is the moment when a mature egg is released from one of your ovaries and travels down the fallopian tube, where it can be fertilised by sperm. It typically happens around day 14 of a 28-day cycle, but this varies widely between women and even from month to month. The egg survives only 12 to 24 hours after release, so timing intercourse correctly is important. Signs of ovulation include a slight rise in basal body temperature, clear stretchy cervical mucus resembling raw egg white, and mild one-sided pelvic discomfort known as mittelschmerz.

Related: Fertile window, LH surge, OPK

An ovulation predictor kit (OPK) is a home urine test that detects the LH surge that occurs before ovulation, helping you identify your most fertile days. Most kits include multiple test strips to use daily, starting a few days before you expect to ovulate. A positive result, usually shown as a test line as dark as or darker than the control line, suggests ovulation is likely within 12 to 36 hours. Some advanced digital OPKs also track oestrogen levels to give a wider picture of your fertile window. OPKs are widely available at pharmacies across Singapore without a prescription.

Related: LH surge, fertile window

An OB-GYN is a doctor who has completed specialist training in both obstetrics, which covers pregnancy and childbirth, and gynecology, which covers the female reproductive system outside of pregnancy. During pregnancy, your OB-GYN monitors your health and your baby's development, manages complications, and delivers your baby. Between pregnancies, they handle routine care like Pap smears, contraception advice, and managing conditions such as endometriosis or fibroids. In Singapore, OB-GYNs practise in both public restructured hospitals and private clinics, and your choice affects cost, continuity of care, and delivery hospital options.

Related: Obstetrician, gynecologist

An obstetrician is a medical doctor who specialises specifically in pregnancy, labour, delivery, and the immediate postpartum period. They are trained to manage both normal and high-risk pregnancies, including conditions like twins or multiples, placenta praevia, preeclampsia, and pregnancies following previous caesarean sections. Your obstetrician will monitor your progress through regular appointments, recommend or interpret key scans, and attend your delivery if complications arise or a caesarean is planned. In Singapore, you can see an obstetrician through the public system at hospitals like KKH, NUH, or SGH, or privately at a specialist clinic of your choice.

Related: OB-GYN, MFM

The oral glucose tolerance test (OGTT) is the standard diagnostic test for gestational diabetes in Singapore. You fast overnight, then have a fasting blood draw before drinking a glucose solution. Blood is drawn again at one hour and two hours afterward to track how your body processes sugar. The whole process takes about two to three hours. If one or more readings are above the threshold, gestational diabetes is diagnosed and your care team will discuss diet changes, monitoring, and possibly medication. Managing gestational diabetes well reduces risks of a large baby, difficult delivery, and longer-term diabetes for both you and your child.

Related: GCT, GDM

Oligohydramnios means there is less amniotic fluid around your baby than expected for your stage of pregnancy. It can happen because the baby is not producing enough urine, the placenta is not working well, there is a leak in the membranes, or as a normal finding close to your due date. Mild cases in late pregnancy may be watched closely with regular ultrasounds and non-stress tests. More severe or early-onset cases carry higher risks, including cord compression, restricted growth, and complications during labour. Your doctor will assess the underlying cause and may recommend increased hydration, bed rest, early delivery, or other interventions depending on how far along you are.

Related: AFI, amniotic fluid

Oxytocin is a hormone produced naturally by the brain and plays a central role in pregnancy, labor, and the newborn period. During labor, rising oxytocin levels trigger contractions of the uterus and help the cervix open. After birth, it stimulates the uterus to contract and reduce bleeding, and it drives the milk let-down reflex when breastfeeding. Oxytocin is also called the bonding hormone because it surges during skin-to-skin contact between mother and baby, supporting emotional attachment. When natural labor needs to be started or supported, a synthetic version of this hormone, known as Pitocin or Syntocinon, is given by IV drip to mimic these effects under careful medical supervision.

Related: Pitocin, let-down

Occiput anterior (OA) means your baby is head-down with the back of their head facing your belly button. This is the most common and favorable position for labor because the baby's head can tuck in neatly and fit through your pelvis more easily. The smallest diameter of the baby's head presses on your cervix, which helps it open efficiently. Most babies naturally settle into this position by 36 weeks. If your baby is in OA position heading into labor, you are likely to have a smoother, shorter labor with less back pain.

Related: OP, fetal position

Occiput posterior (OP), sometimes called the sunny-side-up position, means your baby is head-down but facing your belly button instead of your spine. This makes it harder for the baby's head to fit through your pelvis efficiently. Labor can be longer and more intense, and many women feel strong, persistent pain in the lower back rather than the abdomen. Some OP babies rotate on their own during labor. Hands-and-knees positions, walking, and swaying can encourage rotation. In some cases, the doctor may assist with instruments or recommend a caesarean if progress stalls.

Related: OA, back labor