On average fraternal twins are born between week 36 and week 38 of pregnancy.
This is different than an identical twin pregnancy when there is only one sac being shared and birth will take place around week 35.
A guide to the week by week likelihood of your fraternal twins being born and what it means is shown below.
It is just a general consensus and your own experience will be entirely different but at least it prepares you in some way for the big happy event.
Week 27 - A very small number of twins born this early. High risk but more and more solutions nowadays. Fraternal twins will not usually be born this early.
Week 28/29 - Both fraternal and identical twins can be born this early and will usually only weigh from 1 to 2 lb and parents will be very anxious. Doctors take great care and though you may not see your babies much for the first few weeks (though they do encourage bonding with parent and child for this important early period) the rewards later on will be fabulous.
Week 30 - Now slightly less risk to your fraternal twins as they are over 2lbs in weight and are a little stronger. You will feel steady bumps at this stage too before you feel the pelvic floor pressure. Again babies in ICU as theoritically they are still 10 weeks early.
Week 31/32 - A lot of identical twins are born now. Actually over 10% but less fraternal probably closer to the region of 5 or 6%. Every week is an extra half pound at least and they will probably weigh around 2 and a half or three pound at this part of the pregnancy.
Week 33/34 - Your twins skins has formed a lot better though they will still come out blue. This is normal until the first cry when they turn a more pink colour. Twins born at this stage need care for any defficiencies mum may have suffered herself during pregnancy such as gestational diabetes.
Week 35 - A landmark week. Over 99% of all babies born at this time will go on to have a few weeks of life at least and more. Less health risks and less overall risks of birth abnormalities.
Week 36/37 - Over 60% of identical twins will have been born at this stage with nearly 50% of fraternal twins though this rises as we head to week 37. A quick trip to care for some tests and the babies are yours to bring home. Also mommy needs to stay in to have her stitches or recovery time also. Sounds like this blog is for a child doesnt it?
Week 38/39 - The next 40% of twin pregnancies usually occurs right around now. The mother and babies are united straight away in most cases and babies are a healthy 5 or 6 lb on average around this time.
Week 40 - Still some twin pregnancies are not induced up to this time though more and more doctors are inducing before week 40 now as it can cause a lot of discomfort for the mother and twins are usually as developed as an average singleton by week 38.