You are 2 weeks and 5 days

While you wait, you might wonder how you'd feel if two fertilized embryos were waiting to implant!

261 days until your due date

Have you conceived, and could it be twins? Twins can be nonidentical or identical, and each type is conceived differently.

Nonidentical (dizygotic) twins come from two separate eggs fertilized by two separate sperm. They can also result from IVF if two embryos are placed in the uterus.

Identical (monozygotic) twins come from one egg fertilized by one sperm that then splits into two embryos. The split can happen up to nine days after fertilization, and its timing decides how the placentas and amniotic sacs form. If the zygote splits within the first three days, each twin has its own placenta and sac. If it splits at the blastocyst stage, four to nine days after fertilization, the twins share a placenta but have separate sacs. After day nine, they share both.

Having nonidentical (fraternal) twins depends a lot on family history. It's often said that twins skip a generation, but that isn't quite true. Your chances are simply higher if a close relative has twins, though twins never become certain however many family members have them.

Family history matters most for nonidentical twins on the mother's side, which makes sense because they depend on a woman releasing two eggs in one cycle, and that may be inherited. For unclear reasons, twins on the father's side can matter too; a man may carry a gene that makes his daughter release more than one egg when she ovulates.

Good to know

The lost twin

Twin conceptions may be more common than they seem. Some women unknowingly miscarry one twin in early pregnancy. Confusingly, a woman can have miscarriage symptoms while the pregnancy carries on to term and ends in the birth of a completely healthy single baby.

No one knows how often this happens, or why. In the US, 1 in 31 births is a twin birth, but early scans suggest the figure at conception is much higher. Some experts think 15 percent of all births may start as twins, and losing one could be nature's way of dealing with imperfections.

As a matter of fact

Twin odds

The odds of having identical twins are about 3.5 in 1000.

Some estimate that the chance of twins after fertility treatment is as high as 1 in 38.

Explainer

The uterus lining after fertilization

When fertilization happens, the uterus lining starts producing secretions to nourish an embryo. It prepares in exactly the same way however many embryos implant.

This is general information based on NHS guidance, not medical advice, and every pregnancy is different. Talk to your midwife or GP about anything that worries you. If your baby's movements slow down or change, or you have bleeding, severe pain or leaking fluid, call your maternity unit straight away. In an emergency, call 999.