The placenta is the organ your body builds from scratch every pregnancy to feed the baby, exchange oxygen and carbon dioxide, and act as a filter. It attaches itself somewhere on the inside wall of the uterus very early in pregnancy and then grows along with the uterus across the next nine months.

Where it attaches matters mainly for one reason — distance from the cervix, the opening through which the baby will eventually be born. When the placenta sits well away from the cervix, labour and delivery are straightforward. When it sits very low or over the cervix itself, the cervix cannot open without the placenta bleeding, and the delivery plan changes.

Because of this, the sonographer comments on placental position at every ultrasound — usually at the 11 to 13 week scan, the 18 to 22 week anomaly scan and any later growth scan. Reading "anterior" or "posterior" the first time can be alarming when no one explains, but most of those words simply describe which wall of the uterus the placenta sits on.

Knowing where you are in the pregnancy week-by-week makes the changing scan findings easier to interpret — see what to expect week by week for the normal arc of antenatal milestones.

A placenta is called low-lying when its lower edge sits within 2 centimetres of the internal opening of the cervix, but does not cover it. This is most often picked up at the 18 to 22 week anomaly scan.

On a first reading this can feel alarming, but at 20 weeks the uterus is still small. As the uterus grows over the second half of pregnancy, the lower part stretches outward and most low-lying placentas effectively move upwards, away from the cervix. They have not detached or shifted — the relative distance has simply grown.

A low-lying placenta at 20 weeks needs only a follow-up scan, usually at 32 to 36 weeks, to recheck the distance. In the meantime, the obstetrician will usually advise pelvic rest as a precaution — no sex, no tampons, no internal examination and avoiding heavy lifting — until the position is rechecked.

What a low-lying placenta does not require, in the absence of bleeding, is total bedrest. The old advice of weeks lying flat has been replaced in current obstetric guidelines by sensible pelvic rest plus normal everyday movement.

Placenta previa is the condition where the placenta covers part or all of the internal opening of the cervix in the second half of pregnancy. It is found in roughly 1 in 200 pregnancies at term, and is the main reason placental position is rechecked carefully on later scans.

Marginal previa describes a placenta whose edge reaches the internal cervical opening but does not cover it. Complete or major previa describes a placenta that fully covers the cervical opening. Both belong to the previa family and both usually require a planned caesarean delivery, because the cervix cannot open in labour without the placenta bleeding heavily.

Major risk factors for placenta previa include a previous caesarean section — risk rises roughly two to five times with each prior C-section — a history of previa in an earlier pregnancy, conception through IVF, multiparity (several earlier deliveries), maternal age above 35, smoking and multiple pregnancies such as twins or triplets.

Most placentas labelled previa at 20 weeks improve as the uterus grows. Only about 1 in 10 low-lying placentas seen in the second trimester remain true previa by the time of delivery — but that 1 in 10 is exactly why the rescan at 32 to 36 weeks matters.

Placental migration is one of the most reassuring concepts in modern obstetrics. The placenta itself does not detach and crawl upwards. Instead, the lower part of the uterus stretches and expands during the second and third trimesters as the baby grows. The placenta stays where it is, but the cervix effectively moves further away from it.

Studies consistently show that most low-lying placentas seen at the 18 to 22 week anomaly scan are no longer low at the 32 to 36 week rescan. The exact proportion depends on how low the placenta was to begin with — a placenta clearly overlapping the cervix at 20 weeks is more likely to remain previa than one whose edge was just within 2 centimetres of the os.

Because of this pattern, a low-lying placenta or possible previa picked up at the anomaly scan is almost never an emergency by itself. It becomes a calendar item — a rescan around the third trimester, plus pelvic rest in between — rather than something that immediately changes the delivery plan.

Painless, bright-red vaginal bleeding in the second or third trimester is the classic warning sign of placenta previa. The bleeding can be a single small episode that stops on its own, or it can be heavy and sudden. Either way, it needs to be treated as an emergency.

The two features that distinguish previa bleeding from other causes are that it is painless — there is no cramping or labour-like contractions before it — and the blood is bright red, not brown or pink-tinged. It can happen at rest, after a bowel movement, after sex or apparently out of nowhere.

Any bright-red bleeding in pregnancy after the first trimester needs an emergency visit to a hospital with obstetric services — not a local clinic. Internal vaginal examination is avoided until an ultrasound has confirmed where the placenta sits, because the examination itself can trigger heavier bleeding if previa is present.

If your placenta has already been labelled low-lying or previa on a scan, it is worth knowing the location of your nearest 24-hour obstetric unit from the third trimester onwards, and having a small bag ready in case of a sudden bleed.

Routine placental position is assessed on transabdominal ultrasound — the standard scan with the probe over the belly. This is enough for most positions, including anterior, posterior, fundal and lateral.

When the placenta appears low or covering the cervix on a transabdominal scan, a transvaginal ultrasound is the more accurate next step. The probe sits much closer to the cervix and gives a clearer measurement of the distance between the placental edge and the internal os. Transvaginal ultrasound in pregnancy is safe — the probe does not touch the cervix, and there is no evidence it triggers bleeding or harms the baby.

Many Indian scan reports use shorthand such as "grade 1 placenta", "low-lying anterior", "reaching the os" or "completely covering the os". If the words are unfamiliar, ask the sonographer or your obstetrician to translate them into one sentence — for example, "my placenta is on the front wall and 4 centimetres away from the cervix, so it is not previa".

If a finding is confusing or seems to change between scans, a second opinion at a reputed scan centre is reasonable, especially before any decision about caesarean delivery. See understanding scans, labs and reports for how to read the numbers and which findings need urgent follow-up.

In private centres across Indian cities, an obstetric ultrasound — including the 18 to 22 week anomaly scan and later growth scans — typically costs between 500 and 2,500 rupees depending on the centre, machine and whether colour Doppler is included. Government PMSMA clinics provide a free comprehensive antenatal check, including basic ultrasound, on the ninth of every month at participating facilities.

Many scan reports across India are short and full of abbreviations. They were written for the referring doctor, not for you. It is reasonable, and increasingly common, to ask the sonographer to write or say one sentence in plain language at the end — for example, "the placenta is on the back wall, well above the cervix; no previa".

If a report mentions low-lying placenta or previa, ask three specific questions: how far is the placental edge from the cervix in millimetres, when should the next scan be done, and what should I watch for at home in the meantime. These three answers turn a scary line in a report into a clear plan.

A second opinion at a different scan centre is reasonable when the wording is confusing, when previa is mentioned for the first time, or before any decision about caesarean delivery. If you previously recovered from a C-section, the scar itself is a relevant risk factor for previa and accreta in the next pregnancy and should be flagged to the sonographer.

Reading anterior, posterior, fundal or lateral on a scan slip is reassurance — the placenta is in a normal position. Reading low-lying is a calendar item, not an emergency: most of these placentas migrate upward by the third trimester. Only persistent previa changes the delivery plan, and that change is itself a well-rehearsed plan — a birth plan built around a planned caesarean at 36 to 37 weeks in a hospital with blood-bank support, often with a steroid course beforehand and Anti-D for Rh-negative mothers.

The one symptom that ever truly counts as an emergency is painless, bright-red vaginal bleeding in the second or third trimester. Everything else — anxiety about anterior placenta and late kicks, worry about old scans saying low-lying, jargon in a report — is something a clear conversation with your obstetrician can settle.

If your obstetrician suggests aspirin or extra monitoring because you also have raised blood pressure, that is a separate but related risk — see preeclampsia, pregnancy BP and care in India for what those decisions involve. Otherwise, most placenta-position lines are exactly what they look like once they are translated: routine, expected and normal.