體型適中的胎兒懷孕的時候，醫生在每次覆診都會告訴你胎兒的重量，有否比標準週數過重、過輕？胎兒太大，怕在分娩時造成困難，但胎兒大小的話，又怕生下來後體弱，日後容易生病。究竟，怎樣才叫體型適中的胎兒？
尺碼跟媽媽體型有關胎兒是在產婦的子宮孕育出來，最後經過產道出生，如果胎兒太大，便有可能影響分娩過程，造成出血過多或難產。
足月出生的嬰兒，體重大約在3.2 公斤（約7 磅），如果體重在2.5 公斤（約5.5 磅）或以下，便是過輕。胎兒在出生時的體型，大多跟媽媽的體型成正比，骨架健碩的產婦，多會誕下大BB，而身型纖瘦的媽媽，嬰兒也會較細小。
28 ?36 週生長高峰期在20 週前，一般胎兒的生長速度並沒有大分別，所以可從超聲波素描評估胎兒的大小，以計算準確的預產期。而胎兒生長的高峰期在28 至36 週，到足月時，即37 週後便會慢下來。
要評估胎兒體重，若在週數10 百分位數（10 percentile）之下，就是過輕；在週數90 百分位數（90 percentile）之上，就是過重。在統計學上，胎兒體重生長表並沒有性別的分別的。
<table width="100%" border="1" cellspacing="0" cellpadding="0" style="cursor: default; border-bottom-width: 1px; border-bottom-style: solid; border-bottom-color: rgb(237, 237, 237); border-collapse: collapse; border-spacing: 0px; color: rgb(117, 117, 117); font-size: 0.857142857rem; line-height: 2; margin: 0px 0px 1.714285714rem; width: 624px; font-family: 'Open Sans', Helvetica, Arial, sans-serif;"><tbody><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;"><p style="line-height: 1.714285714; margin: 0px 0px 1.714285714rem;">懷孕週數</p></td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">胎兒長度（厘米）</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">胎兒體重（克）</td></tr><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">0 - 3</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">0.7</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">--</td></tr><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">4 - 7</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">2.5?3</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">4</td></tr><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">8 - 11</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">9</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">20</td></tr><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">12 -15</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">18</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">120</td></tr><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">16 - 19</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">25</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">250</td></tr><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">20 - 23</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">30</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">600</td></tr><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">24 -27</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">35</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">900 - 1,100</td></tr><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">28 - 31</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">40</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">1,200 - 1,500</td></tr><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">32 - 35</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">45 - 47</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">1,700 - 2,300</td></tr><tr><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">36 - 39</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">50</td><td style="font-family: Verdana, Arial, Helvetica, sans-serif; font-size: inherit; margin: 8px; padding: 6px 10px 6px 0px; cursor: text; border-top-width: 1px !important; border-top-style: solid !important; border-top-color: rgb(237, 237, 237) !important;">2,550 - 3,050</td></tr></tbody></table>text: Adi