24 week fetus

how big is a 20 week fetus

At 20 weeks the NHS figure is about 25.6 cm from head to heel. That is the week many people treat as halfway through a 40-week pregnancy, and it is when an anomaly scan is often offered. From this stage, length is commonly taken head to heel. In earlier weeks the legs are curled, so the usual scan length is head to bottom (crown–rump).

A 24-week length of about 30 cm and a 20-week length of about 25.6 cm are therefore on the same kind of tape. Comparing either number to an 8-week or 12-week figure without noticing the change of tape is how a growth chart looks like a jump that the baby did not make.

These lengths are typical chart values, not a verdict on one pregnancy. A scan report from the clinician in front of you outranks a web page. This page is a count of weeks and published chart sizes, not care.

Eight, ten, twelve, and fifteen weeks

Count gestational weeks from the start of the last menstrual period unless a dating scan has reset the clock. The week number in a title is that count, not “weeks since a positive test.” 8-week fetus: NHS about 16 mm, still a head-to-bottom measure. Around this week the embryo is described as becoming a fetus.

10-week fetus: NHS about 30 mm from head to bottom. 12-week fetus: NHS 5.4 cm from head to bottom. Dating scans are often offered in a window that includes this week. 15-week fetus: NHS about 10.1 cm from head to bottom.

Those four numbers are not on the head-to-heel tape used at 20 and 24 weeks. Read them as early lengths. Fruit nicknames on the same pages are nicknames. The millimetres and centimetres are the count.

Second trimester is commonly treated as starting at week 13. Twenty-four weeks is still in that trimester. Forty weeks is the usual dated full-term mark on the same counting system, which is why 20 weeks is called halfway.

If the tape changed from bottom to heel, the baby did not leap a fruit size overnight. You switched rulers.

What the week number is counting

A “24 week fetus” is 24 completed weeks of gestational age on that dating system, or the week labelled 24 in a chart. Charts differ on whether they mean the start of the week or the completed week. The scan measurement is a length or a set of lengths on a given day. The week number is a calendar label. Keep both.

NHS notes that from 24 weeks a baby is considered viable, meaning survival outside the womb is possible with the right support. That is a clinical category, not a size in centimetres, and not a promise. Size still varies. Care still belongs to a midwife or doctor.

If you are comparing two week numbers, subtract them as weeks, then remember each week is seven days. From a 12-week dating scan to 24 weeks is twelve weeks of calendar, not twelve millimetres of growth. Growth is the scan. The week is the clock.